One of the common issues with Kontakt and MainStage was running Kontakt in MS with multiprocessor support enabled within Kontakt.
Is this the same for GP and Kontakt or does it make no difference?
Multi-processor support seems to be enabled by default for AU version of Kontakt. We have been using Kontakt from the very fist days of GP development and it works very well.
I suggest you give it a try yourself - download Gig Performer and try it out for 2 weeks. There are no limitations during the trial and you will be able to test everything you need.