You might want to review this thread as this topic has been discussed a lot.
There is also an article from our blog that talks about how to migrate slowly from one plugin host to another over time.
As for Omnisphere, I have found that a few instances of Omnisphere tend to just chew up cycles no matter what host I use. If I really need to use an Omnisphere sound on tour, I sample it into Kontakt, which is my goto sampler for everything.
But certainly, we do encourage everyone to test Gig Performer thoroughly before purchasing to be sure that it can do what is needed for them.