Cobra. You seem a little confused. Thrust is from diameter which you have alot of, speed is from pitch which you have little of. You'll be lucky to get 100 mph.
What you need to do is decrease diameter and sacrifice thrust and increase pitch so you are not getting passed by trainers. If you dont want to cut props youll need to buy them of the right size.
7.5x8 is a place to start. It will still come on pipe and give you potental for 150+ mph and 5 pounds of thrust for takeoff.