well. you've not said which version of MS you're using, but... I would
get version 2.2.2 for MainStage multicore support.
this one sounds weird, but if you have MS v2.2.2, try telling Reason *not* to use multiple cores, let MS handle that
up your RAM to at least 8 GB
lose Reason if you can (nothing wrong with Reason, it's just two things to deal with, Reason *and* Rewire)
run the "lite" version of patches in Omni, and follow some of their advice for lowering the CPU and streaming load...
I'd go SSD for your startup drive AND get one of the kits that replace the optical drive and put an SSD there for sample libs.
Omni doesn't have to be a CPU hog, it just usually is, because everything in there is so dramatic, dial that in a bit and it'll fly pretty well.
Logic v9.1.8 (64-bit), MainStage v2.2.2 (64-bit), OSX v10.8.2
MacPro 8 core 24GB RAM, 2x 23" Cinema Display
MacMini i7, 16GB RAM
retina MacBookPro, 16 GB RAM
MacBook Pro, 8 GB RAM, MOTU 828, GlyphTech ext drive
MacBook Pro, 6 GB RAM