I think it requires some acting skills to convey authority and evil while shouting through a plastic helmet. The other actors need someone to react to and interact with. Something Lucas forgets about sometimes.
There is certainly no doubting Peter Mayhew is giving a performance and not a set of moves. They tried shooting scenes with his stand in on Empire when he was physically exhausted from carrying Threepio around all day. They could only do long shots because the character just wasn't there.
And just for fun, here's Prowse as a superhero very well known in England.