I got myself some shiny new X.0 shifters and derailleur, can I re-use my existing (Jagwire) cable outers, or is it better to replace?

If keeping them do I need to attempt to "clean" them internally for smoother shifting, incase there is any crud inside them?

Probably easier just to replace.

It'd just be tight of you not to after spending that much on the shifters and mech. They don't just get dirty inside, they wear internally too.
Make sure you get sealed ferrules for them. Shimano's have O rings in them to keep the crap out.

and you all know what my next question will be!

What should I get? (thats lightweight)

unless they have an inner liner that can be replaced (as per Nokon's) I'd replace.. With SP41

You could try grey outer casing, that LOOKS lighter than black. I'm not aware of any specific lightweight outer on the market.

Transfil flying snake stuff is made from Kevlar but it's £22 for a set. You'll probably save 10g!

Ive just been looking at that Nokon stuff, I personally think its FUGLY!!!

I am not too bothered about saving weight, just dont want the outers to weigh any more than my current Jagwire. Do quite fancy some red outers though…. 🙄

I just use bog standard shimano stuff from the LBS (JE JAMES in sheff). I run full length outer, so just get them to cut me off 3m for a bikes worth.

Ive just been looking at that Nokon stuff, I personally think its FUGLY!!!

I bought it, it's shite! Magazines have a lot to answer for.