Just for the heck of it, try an 11x4 on a .40, a 12x4 on a .46, or a 13x4 on a .50. More thrust, not much less speed, and MUCH more pleasant landings!

You are not going to over-rev the engine with these props. I have run stock Super Tigers over 20,000 on a tuned pipe without problems. Balance the prop, though.

Try those low-pitch props and see how much more fun it is.....