The chances are that a fast lens, shall we say a Nikon F1.4 is made to the same standard as a Nikon 1.8 so the difference will be well nigh on impossible to distinguish except on an optical bench. We are really into straw splitting territory here.

If you are going to stop down the former to say F8 and the latter to the same apperture, whats the point of paying extra cash for the wider apperture lens.