I used Giga 3 with Sonar 4 for a while when I was still on a PC. Didn't really have any issues with the setup, but I used the Giga-VST adapter from fxmax instead of using rewire (more stable, better results and allowed me to use more than 16 channels). So the setup should definitely work, though you may want to ask in the Giga forum to see if the hardware in you computer with play nice with Giga. Also, like people mentioned in the your other thread, you may want to look at GVI so you can open it as a plug in rather than having to deal with the standalone version.
Thanks everybody for the helpful replies. After reading them all, I'm going to hold off for the time being. I have Halion 3 which originally gave me trouble, but the latest update seems better able to cope with gig files. I'll experiment with that before I make up my mind.