Not right thread for such questions IMO, but
:
1) max workgroup size remains the same.
Look into stderr of AP task, it liss device parameters.
2) yes, algorithm can change indeed. For example, look my post about why NV MB build should not work with workgroup size of 32.
[more technically, kernel aware of its launch parameters, in particular, about workgroup size, via get_local_size() call.
So, surely it can and sometimes will behave differently for different workgroup sizes]