2.3 on the front and 2.1 on the rear – Not UST but run tubeless on stans 355 rims, will it be a problem, what are they like grip wise?
As grippy as my nevs? Fragile? Any other comments?

Been running a 2.1 on the back (also non UST as tubeless), pretty good so far, but not had it for masses of time yet…

What pressure?

I have 2.1 supersonics on my canal commuter bike, I can't imagine they will be anywhere near the grip of your Nevegals IMO. Mine sometimes have a mind of their own on rutted wet earthy sections.
EDIT I have mine pumped up pretty hard for speed.

They are ok but muddy corners are a bit oh that was close.I really don`t think you can compare the two tyres as they are different.They are fast though and the 2.1 are narrow

they are not comparable with nevegals, which i found heavy and slow

the speedkings found a surprising amount of grip for the tread pattern, light and fast – good for 3 seasons round here.

where is "round here"?

Speedkings are fast, I had nevegals/highrollers on my canal commuter before buying the superlight speedkings. The difference for rolling speed was very noticable.

rode with 2.3 front 2.1 back for the last two years, excellent predictable, fast. All year round use here at Drumlanrig and occasional jaunts onto the hills, puffer and sanitised 7. good in mud/loam/roots as they clear quick, i run mine quite soft front 25-30 rear 30 ish but harder for rocky/faster rides. I'm running tubes but my mate runs stans and is convinced.

Rode mine on various Stanes, it did surprisingly well for both grip + robustness (considering how fast they are). Mostly ran it at 40psi.

I use SK supersonics (black chilli) as an event tyre. They're crazy light and make you feel super fit!

They have an impressive amount of grip, roll well but wear quickly. The growl noticably when about to loose grip. Their only real weak point imo is on wet rock where it seems random when they let go. They deal pretty well with mud.

I run them ghetto tubeless at lowish pressures. Never had a puncture (in 3 worn out ones). The 2.3 is about the size of 2.1 for most other makes (schwalbe/maxxis etc). I never had a 2.1 SK.

Nevegals ime are good but a bit slower rolling and have thicker sidewalls and way heavier which is good for rockier places. But there are lot's of different versions of these too.

Horses for courses etc!

Ran SKs (black chilli) on my Yeti ASR and ran them on my light Trail HT, very fast running, I like them, but quite fragile and puncture relatively easily. Grip pretty good, I used them as an around tyre, Devon, mud in winter, dry sometimes (like now!!). They breakaway faster and in a more irregular fashion than my other tyres, (Specialized on my SS and High Rollers on my 6" heavy trail but over all worth the trade off because of the light weight and rolling speed.

So more fragile (than most), irregular breakaway (but ok) but FAST.

I think a good/better compromise on trails are the Mountain Kings, little less fragile/thin, slightly less irregular breakaway and only just slower.

Also I would not run the 2.1s My 2.3 are the narrowest mtb tyres in the house.

Have run them a fair bit, they are my favourite tyre generally, although in really heavy sticky mud they become pretty useless. Do wear quickly, but are fast and I haven't had particularly bad issues with fragility (I'm not exactly a lightweight either) BUT puncture puncture puncture every ride without fail. My record is 5 in one 2 hour ride. I got so hacked off with punctures that I took them off a few months back and am now wondering what to do as winter seems to be over.

Haven't used them myself but several riding friends have found the sidewalls very vulnerable to cuts – there's a fair bit of flint locally, which rips through them extremely easily. Depends on your local riding really.

I have a set – good in the dry. fast rolling but wear very quickly on tarmac.

Have recently put the OEM 2.1 set back onto my Boardman after using Mud-X 2.0/1.8 through the winter. Was running tubed at about 20/25psi with the Mud-X. Similar pressure on the Speed Kings pinch punctured so will be going up to about 28/35 and see how they work at that, and also switching the front 2.1 (which in reality is between the 1.8 and 2.0 Mud-X in size) for a 2.3 for some adding grip and resistance to pinch punctures. They definitely roll quicker than the Mud-X and they're pretty decent in the mud and wet. Hard to say if they let go more quickly than the Mud-X because all tyres are more snappy in the dry but they have a surprising amount of grip. I have a theory that the relatively tall flexible side blocks deform under usage hence them running well at higher pressures, with the treadblocks as well as the carcass moulding to the terrain.

If I was riding in the rocky north then I'd want something much bigger in volume that could be run safely at low pressure for maximum grip and minimum chance of snakebites but they seem a good non-winter tyre for the chalky dusty/muddy rooty singletrack and downland round here.

Back tyre seems to wear relatively quickly. They're rubbish on tarmac, slow and noisy due to the widely spaced knobs.

What else is out there that is a quick rolling tyre which can still handle some mud and doesn't cost a fortune? Once this set has been worn out am up for some experimentation.

I certainly found them virtually useless at the first hint of mud / water / wet grass

On the flip side of TJ's finding, I find them very good allround, no its not a mud tyre but I rekon they cope very well, the wide spacing means they clear, but the lower knob height means they dont quite dig as deep as a mud tyre would..

Nothing grips on wet grass well short of a snow spike!!

Do On One still sell them for a tenner?

one one are knocking them out for 15 at the moment, they also do the michelin xc/aT 2.0 for a tenner, I had a set on my last bike and they were great (for £10 each) although struggled when cornering really hard at muddeglla – which could have been the very cloying mud there it turns all types of tyres to slicks…

Fast fast fast. Did I mention fast ?

But IMO only good for hartd compacyted dry trails

Just put a 2.3 on the front of my bike. Slightly different tread pattern to the 2.1s (2009 vs 2010?) and the knobs look marginally lower profile (just measured them, they're not – 5mm high for both). Big difference is that the side blocks on the 2.1 stick out to the side (hence them appearing so much narrower than they measure) well beyond the carcass whilst on the 2.3 this is much less pronounced. Once I've worn out both 2.1 rear tyres I think I'll try a 2.3 on the rear, the extra volume looks worth having and when winter comes I'll be going to mud tyres anyway so the skinniness is of little benefit.

2.3
carcass: 50mm(1.97")
tread: 55mm (2.17")
carcass height from rim: 45mm

2.1
carcass: 44mm (1.73")
tread: 52mm (2.05")
carcass height from rim: 40mm

Not quite sure how wide a rim you'd have to put these tyres on to get them to measure their claimed width!

Another pinch puncture on the remaining 2.1 Speed King at the back (though previous snakebite was at the front), despite running higher pressure, around 35 psi. 2.3 Speed King is much bigger in volume (about 30%) so shall get another and stick it on the back to hopefully reduce the change of pinch punctures. Will order a folding one this time to see if the compound is any different. Geekery…