I'm getting larger converted files with Studio Premium than with HDLink. This is about 41% larger for high quality, and about 32% larger for medium quality. I wonder if I-frames is turned on with Studio? Is there any way to turn this off?

I believe it is I frame only, however I frame vs IP was not that much different, add the difference reduces with higher quality modes, you are suggesting the opposite. Please rerun your tests with Studio and the same source through HDlink. If you are seeing difference like that please report to support.