I've just bought the upgrade to Nebula Pro. I use Sonar 8.3 64bit (latest version is 8.5). Win7 x64
With the same program loaded and same nebula settings (testing with non-reverb version) Sonar CPU meters show the x64 version uses much more CPU. In this case 32bit uses about 0-1%, 64bit uses about 6%.
I did the same test in Reaper x64 and there is no CPU difference there. Must be a Sonar problem.