I've found this combination to be pretty sharp, especially considering there's a 2x.
But I have noticed that if I am in continuous focus mode, with SH2 at maximum, even though a subject may not be moving very fast at all, sometimes the images are a bit out of focus.
Has anyone else encountered this? Is a solution to step down from 50 fps to 25 or even 10? Or maybe leave at 50, but do single focus and rely on depth of field?
I think the focus issues may be more that you have a 2x teleconverter attached rather than suspecting frame rate or subject movement. Even a 1.4x converter will adversely affect focusing, and a 2x even more so.
Regarding sharpness: I have found the Oly 40-150mm f/2.8 Pro + MC-20 to be a finicky combination. I finally came to the realization that as long as I'm focusing in the mid range (closer than infinity), I can get good sharpness with the 2x. Of course, with that kind of magnification, camera technique is paramount.