My educated guess is that this lens is too slow to work well on the Letus Extreme. I have a Letus Extreme and I can't stop down past f4 without seeing the ground glass. I have an f4 200mm that does not work with the Extreme. How fast is your 70-200mm? You would probably be okay with a faster lens (or with a better lens adapter).
I think there should be a lens adapter forum on the cow! I suggested it to Ron once.
Jason Jenkins
Flowmotion Media
Video production... with style!