Due to the level of slop and reduced traction on the forest roads round here, I am thinking about putting a fatter, grippier tyre on the back. Plan would be to use g-one bite with 29×40 front and 27,5×54 rear (should help with comfort as well)
Anyone else done this?

Currently building a rear 27.5 wheel, have a 700cX40 front tyre & 27.5X47mm rear tyre for my AT Tripster

More grip on the back than the front. Are you sure that’s what you want? Why not 650b both ends?

Although I’m not sure wider tyres do necessarily bite better in slop

Where I am riding it’s more 1cm dirt on top of wet, loose gravel/fire road so I am trying to get a combination of not sinking into it and not spinning out on climbs.

I’m not convinced it’ll work. For one thing narrower tyres are usually the preferred option in mud as they bite into it. Bigger tyres tend to be reserved for rougher tracks.

IME bigger tyres (all the way upto fat bike tyres) behave differently, but don’t necessarily offer more grip in all situations. If you have two similar wheels f+r then the balance point between them tends to be predictable in most situations. Especially once they start to slide, you can catch a CX tyre when it slides, but they seem a lot harder to drift with any control. Whereas the fat bike is so stable you can pedal it out of a slide.

If you can get a pair of 27.5 X King 2.2’s (the old ones come up a bit narrower normally) you’ll have grip and speed. They offer alot more grip for not much more resistance. Pretty quiet.

I don’t like different tyres front and rear at high speed on loose high speed corner decents on gravel. Having the front holding and the back not at 25 mph is unnerving. Like it’s articulated in the middle.

Just fit more grip and keep it narrow to punch down through the slop. Forecaster’s are supposed to be good for this stuff, but not tried em. Probably easier tubeless too than the Conti’s (which are reluctant fellows usually).

That’s a good point my fat bike will ride through any mud fine the tyres are great in mud and as you say very stable on the other hand the Bonty mud CX tyres on my cross bike are good but will slide around but the tracks I use with that bike you are through the mud so much faster on the flat than the fat bike you stay upright fine and its a nice feeling slightly not in so much control if that makes any sense!!

I’d focus on how knobby the tyres are rather than width.

I put wider tyres on my gravel bike (45c) and its noticeably worse in mud.