Would there be a hit? C++ has to do this:
object pointer -> load vtable -> load function pointer
while a compressed fat pointer would be like:
fat pointer -> shift to extract vtable index -> shift to make into a global table offset -> load global table base from a global variable -> load function pointer
Two loads in both cases, but in the compressed case the first one will almost always be cached; more code bloat, but not that much, and you save a bit of memory in the objects themselves.
It should be possible as a compiler option if you are compiling every object that you link to and statically linking them into a single binary. Even then there's be a performance hit.