Would a stock XB8 clutch grab well with a Novarossi Rex XR .28 or should I invest in a Fiorini or Werks clutch. I can't decide if the Fiorini sliding clutch is better than the Werks, I haven't heard any feedback on either one.

the stock XB8 clutch is very similar to the Mugen Alloy clutch & is more than capable in any buggy related application
try it with the 1mm springs first & go up or down from there depending on the desired bite or feel

I use the Mugen 1.1 Springs and the Mugen Carbon Clutch shoes on my Jammin X1CR Pro and I like it a lot.

So aftermarket clutches aren't really necessary? If the stock clutch is good enough I sure don't want to spend 80 bucks on an aftermarket one.

Spending $80 on a clutch is absolutely not something you need to do.

Kudos to that