Virtual instruments have never liked Pro Tools, whatever the version.
Maybe some crossed wires here in that sometimes "we" talk about standalone and the AAX (or whatever other flavour of plug) in the same thread, and they work differently.
RAM affects the standalone and i've tested it, and obviously the more you have the better. I found significant differences with the intel macs, one with 8Gb and one with 48Gb - But that goes without saying, right?
With 8Gb of RAM i found i could not really play it without it cracking or freezing under about 512, although that still has issues. Increasing to 1024 solved that. With the 48Gb machine, I could happily go to 32 and it not affect the sound.
As far as the plug in inside a DAW, there's so many things that affect it. Buffer, playback settings, track count, Core count, other plugs, voices, delay compensation, even having the bloody plug in GUI open.
Again, with the same two machines I had totally different experiences with the exact same set up of Pro Tools running the same session.
Playback / Mixing you can easily take it to 1024 and be happy that you are going to play everything fine, I've never had issues there. Recording and trying to get decent latency is different.
From my tests, the main reason this is so is due to having the GUI open, closing the plug GUI when playing results is a far smoother experience.
Thanks, Stuart