It sounds like your,re arguing against yourself there. If the only advantage humans have over most animals is brains and thumbs then why burden a subrace with our weaknesses when adding thumbs and a better brain to something more animal would be easier?
Physically speaking there's much more involved in trying to make someone stronger or faster than there is for making something stronger or having thumbs.
To make someone stronger or faster you have to modify the muscles, the bone density and configuration, the heart and vascular system, as well. No matter how much muscle you pile on someone and how fast you make the twitch responses they aren't going to be able to beat a cheetah because our frame is just not built for speed. Animals on the other hand already have a brain and the physical advantage. Making the brain larger and adding a thumb aren't nearly as involved.
It's much easier to take a rally car, add a minifridge and allow the seat to recline than it is to make a motorhome perform on par with a rally car.