I flew a similar setup to what you have and I needed full throttle for a knife edge.

If you are having trouble regaining speed after a turn you want to stay with a low pitch prop. With my Dolphin I used larger wheels and a piece of plywood under the gear to fit a larger prop. If you want to do aerobatics, you want at least an 11" prop, if not the 12".

Did you laterally balance your plane? Are you sure things are fastened properly (battery and receiver)? Just some things to check.

If you're slowing down in corners I wouldn't go to the 11x6, it will take longer to pick up speed. 11x5 is a good prop (I found anyway) on that engine if the 12x5 doesn't work for you.