Are we done with the cast is old and overweight jokes yet? :/
CGI is all in how you use it. The Narnia movies have had guys in creature suits with the facial expressions augmented by CG. Beats the pants off of interacting with the proverbial tennis ball on a stick. ;)
I still can't think of a good reason why there couldn't have been at least one physical clone trooper costume made for the prequels.