Your trainer is never going to be a speed demon, regardless of the prop & the smaller diameter & steeper pitch props are literally wasting their efforts so you aren't getting the most out of your engine. I suspect that you don't have sufficient ground clearance for either a 12-4, or a 12.25-3.75 (the best prop choices for your plane for aerobatics), so an 11-5 is a good compromise, & an 11-6 will also work OK at the expense of some climb (vertical aerobatics).