Have you tried some different props to see if they might give you performance more to your liking?

I've got an Irvine .53 on my Ultrastik, and I think it's plenty. A .90 is a lot of engine, and probably far too much for the airframe unless you do some serious reinforcing. Somewhere along the line you get to the point of rapidly diminishing returns, where the weight penalty kills the benefits of the extra power.