RE: nice shrike 40 mod
The power/rpm boost you get with the ultra-thrust or jett-stream far outweighs any drag delta beyond the stock exhaust system.
Razor is correct. The AX does not like rpm as much as the old 46FX. Stick with the higher pitch props - keep it in the 12,000 - 13,000 rpm range with the stock exhaust.
If you want something to put the fan engine in, look up Dan at Carolina Models (look in the vintage pattern forum). Get ahold of a T2AMII or brushfire or something similar from the 'good ole days' of pattern.