These are both good things to read (I started both of them
). My thundercat
45mph is about right for a 15p bunny prop. Maybe a bit slow, I was getting 49mph with my 15p prop, although it was cupped.
Once you start going for speed you really want a surface piercing propeller, chopper or cleaver.
The best starting point is trying to get max rpm with the prop you've got.
You should be getting more revs, the limiter is set at 5850. To get more rpm raise the engine up. If you're prop is cavitating you want to get it cupped for more grip.
Most of your performance gains are done by propeller selection. If you still can't get the speed you're after then start engine modifications.
Saying that, a set of boyesen carbon reeds is good on any engine. If you want more noise as well as speed you could do the exhaust mod - a 1" hole in the gearbox so the exhaust gases don't exit through the prop. Lighting coils can be removed from beneath the flywheel, frees up some hp.
Try and get some photos up of your propeller, engine set up, trim etc.
You can see here I ran a bit of orange string across the bottom of the hijackers to give an idea of prop height. This was with an 18p chopper, a 15p bunny should be lower.