are you running in 64bit or 32bit? Im almost sure if in the 32 you can only utilize 4gigs of your ram.. I may be wrong, but i think this is the case.. For this reason, I stopped using PT10 w/ reason rewired because large sessions just couldn't handle it.
I now use Logic 9 w/ reason and run 64bit mode (PT doesn't support this) and utilize all my ram if needed (16gigs).. No problems so far
My computer is a Macbook Pro 2.6ghz i7 quad core with 16gb of ram