also, consider that the 13 x 7 is a torque prop and thus your performance should be fine. you shouldn't be doing any speed runs anyway! what i've noticed is that as the engine unloads too much at speed on that prop so when you're going fast the loading on the engine between 70% and 100% is minimal. but if you're pulling out of a hover, 70% and 100% will make a big difference and that's why when you're on the ground you can hear the difference because it's static.

now if you use a 10 x 10, you'll probably hear the difference, particularly when the airframe disentegrates :-)

good luck!