I mainly use MainConcept AVC/AAC, but during my frustration of trying to get Vegas Pro to work with my HD7950, I have also tested it on Sony AVC/MVC, and XDCAM EX which are the only ones that produces mp4.
For some reason XDCAM EX renders fast, almost 100 fps but gpu usuage is still near idle and cpu is still near 99%. The problem with XDCAM is that it doesn’t compress the video as much as the other encoders so my video size after encoding is 2-3 times the size if i had use MainConcept AVC/AAC.
But now that you mention it, I’m going to test it on every encoder available in Sony Vegas Pro 12 to see if any of them can use my gpu.