Looking to either upgrade my 2017 stumpjumper carbon expert with carbon wheels, or pick up a 22 stumpjumper pro. I know carbon wheels increased performance on my road bike, do carbon wheels improve performance on mountain bikes?

Yes, in the same way for the same reason.

Adoption is not universal. A crash bent aluminum rim is something you can maybe bend back, maybe ride out, but a cracked carbon rim is done. However it's really really hard to do that

How do you measure performance?

I have carbon wheels on my hardtail MTB. They are light, which results in faster climbing. They are compliant, which results in a soother ride. I broke a spoke in the rear wheel, and the rim stayed true enough for me to do 5 miles of downhill at normal speed, and finished my ride without having to walk my bike home. I'm a fan of carbon wheels.

The older Mavic Dee Max Pro and the newer Dee Max Sam Hill wouldn't break like this ,but impressive torture test of a carbon wheel.

I just ordered a set of Ibis S28 carbon wheels, on sale at Ibis. They are roughly a pound lighter than the OEM Specialized wheels that came on the bike. As such you've got a weight savings on the bike and in theory should see the difference when getting up to speed as the lighter wheels will take less effort to get rolling. Thats's a big advantage on a mt. bike, maybe more than on a road bike as on a mt. bike you are constantly slowing for turns, then accelerating again on tight single track, or so goes the theory. As well can be stiffer laterally, so you might notice the greater control when turning. Again, so goes the theory, but when you research these wheels, that's generally what kind of feedback I read.

Steve of Hardtail Party on YouTube swears by a few premium name brand rims that are more compliant. He thinks it matters more on a hard tail. In the next breath he will say it's fine to use the harsher ones (which includes all the Chinese ones) on full suspension bikes. I'm not convinced, it seems like a real princess and pea situation when you consider the tire. Maybe it's more NVH than performance per se.

I’m using Enve 730’s on one of my bikes and admit they feel too stiff for me

the 7 series wheels are burly enduro spec wheels and I really should have sourced a lighter duty set. I’m a heavyset gentleman and figured the heavy duty wheels would be in keeping with my needs but turns out I’m casually looking for a decent set of aluminum wheels for the rig. I is to the point that I have to air the suspension down and it started getting a lot of strikes and running way too much sag in the open setting

But that said I think they would be great for an aggressive rider who is in shape. Which is probably what they are designed for anyway

Could probably buy four nice pairs of Stan’s from the proceeds of selling off the envies

FME, carbon wheels are fine, but not necessary since I don't expose them to the rigors that would compromise a similar weight aluminum rim. Stan's have been adequate and much less expensive.