I have the same plane, haven't finished it yet, but my guess is you might be happy with 3 1/2 inches rudder. Some say there is no such thing as too much rudder. JR does make longer arms, maybe you can put the cables on a shorter position? Anyway that will probably help, but the whole game is about practice and reaction time. probably 90% skill, 10% set up.

Just took a closer look. The firewall is alreay set for a couple degrees of engine thrust, you might not need any more. this plane was designed by a 3D dude for 3D dudes. chances are high that if you just practice a bit more, the tendancy to fall to one side will go away (mine always "pitched to the belly", untill i got the hang of it) could just be your thumb is faster going left that it is going right.

Hang in there