A tach is a must when playing with outboard props.
Theres a "tiny tach"that can be installed aftermarket.
If your motor can get the proper rpms with the 15 pitch, it would go faster.If the motors rpms are to low,not only will you go slower but will end up hurting your motor(lugging it).
RPMs are key,thats why a tach is pretty much a necessity.
A quick way to think of prop pitches is for every turn of the prop,whatever the pitch is how many inches you should move.
So,1 revoltuion with the 11 pitch=11inches;15 pitch=15 inches.
This isn't exact as slippage and other factors come into play.
So,you may be able to turn a 15" prop and that would make you go faster.(lugging motor just as bad as to much rpm)
Slower to plane,may not without moving weight around.
The 11 pitch would make you jump onto plane quickly,good for heavy loads but you may go over the suggested rpms at WOT.(high rpms bad also)