All I can tell you is that a 42 does a very nice job. I guess your 30 would spin an 18 x 10? I'm using a 20 x 10, and the two engines would be at the same RPM ... so you are losing quite a bit.
It depends on the flying you are doing, also. I have a 52 in a H9 CAP with a 80" wing, and its performance is like this one ... unlimited vertical. I am sure the 30 would fly it, but you would not get 3D out of it, I don't think.