I thought I would slide this over here as not to tie up Bobber's thread on his speed ship project......
To answer a few questions......
I have not received a whole lot of info on the RE engine installation. The FIRE 90 was being installed with a riser header, with the header exiting the cheek cowl just like and old F1 pylon racer use to (r/c type). He was building his own version of the Jett/Ro-Jett header muffler for it to quiet it down and give a performance boost. Last I heard he was testing the header on the test stand.
On that Rossi RIRE installation mentioned..... probably will work ok.... but the same RE exhaust issues come up. Some sort of mini-pipe might be in order there. In general, the airplane is not all that condusive to any exhaust arrangement. The 4C exhaust is sorta 'cheating' in my book
About speed..... although there are a variety of variables involved..... quick calculation using an 11x9 or 12x9, 9" pitch running at 14K rpm comes out to about 120 mph at near 100% eff. Add in a slippery airplane, assume the engine unloads a bit above that... perhaps a shallow dive or tailwind..... and the 120mph mark is about right. What this thing really needs is a narrow, thin blade 12x10 that will get up to 13.5 on the ground.
With the Toni muffler extention Dub has available, its ok to turn the Jett down lower rpm a bit (mid 13K range) on the ground. It will haul the 11x9, 12x8 or 12x9 prop ok with the 'longer pipe' set-up, and will unload up to song in the air.
Getting the airframe to move up into the 150 range will require a bunch more power. Something that would turn a 12x12 up around 15K might get you there.