I recently installed a 55 ax in my extra 300 and I broke it in by the book with one tank of fuel then tuned it and went flying. It may need retuning after a couple tanks of flight, but it won't hurt it to go flying after one tank of break in. I used a 13x7 and 12x7 and found that the plane has more speed with the 12x7. I think the 11x7 won't be enough prop for you, I would go up to a 12 incher at least. I run an 11x7 on my TT pro .46 and it screams. Experiment with 12x6 and 12x7 or even an 11x8. A guy at my field has a very fast venus with a 55ax and 11x8 prop.