It's really going to depend how many tracks you're using. Currently I only have V6, but I do have Alalog Lab 5 (or should we call it 5.5 now), and I find it way more processor intensive than previous iterations. If you create a lot of tracks with lots of additional processing, it's possible you might have to freeze / lock a few tracks (or whatever phrase your DAW uses for bouncing the midi track to audio, so it's only streaming audio audio from your drive, not processing everything in real time).
Certainly I don't think you'll have any issue running an VC9 synths one at a time. If your computer couldn't handle Roland Cloud's Jupiter 8 I'd say that's more an indictment of Roland than your computer. Your computer should be plenty powerful enough.
How about downloading the time-limited demos and find out?