The findings that Mountain Lion seems to do a lot less page outs (or even none at all) suggests it's making much better use of RAM than before, provided you've got plenty of it at least.
People on stock RAM may find they don't get much, or any, difference. There aren't really any details out about low level changes in Mountain Lion so it's hard to say for sure, it's possible the changes favour higher RAM configurations more than older versions?
I do agree that multi-threading is still a bit wonky; I have particular problems with very high numbers of TCP/IP connections potentially causing multiple apps to lock up (those using TCP/IP that is), but that's nothing new. There also stills seems to be a ton of stuff going through the kernel_task process which is something that I would have thought could have been improved, since it can act as a significant bottleneck for certain combinations of applications, and it seems anecdotally a bit worse in Mountain Lion (that process seems to have even more work to do), though I can't really pin down why that might be, or if it's my fault
Still, I think most responses are favourable, with many getting a smoother experience with better use of RAM (so long as you ignore what Activity Monitor says). Some of the new APIs and updates to existing ones are very interesting for possible future performance as well.
I think that one thing that's important to remember is that Apple has already done a ton to improve performance of OS X since v10.0 first hit. While I'm sure there are definitely some further improvements to be gained, you kind of have to wonder if it's realistic to expect much in the way of further core improvements? Performance losses though are definitely a cause for concern if they affect older machines, but if you're not one on of Apple's current favourites then it's not always safe to assume updates will be better for you!
