The Aresti does not do that great a KE but its not lousy either. Take your CG back a bit, mine was with a Saito 72 and I had about 2 oz of lead in the tail and I could have taken the CG further back without the plane being twitchy. My WM Super Chipmunk does a much better KE than the Aresti. There is no need to have a larger rudder.