RE: looking for speed model
Wow, the 75AX is not going to be anywhere near 18K, not that I have seen or read anyhow. Sorry man...I agree the 13/10 is WAY too big for that engine but it needs to be made clear that you will most likely not get similar performance to the Jett's. I would try a 12x8 or 12x9 apc on that engine.
The AX similarly to the Tower-75, generates it's HP at lower RPM's. You can get some impressive speed based on prop. selection but I would not spend the $ on a tuned pipe.