What fuel and glow plug are using. What rpm is the 91 FX turning the 13/8 at?
Keep in mind the a 13" prop provides higher static thrust numbers than an 11" or 12" prop ever will. It's simple math. This heavy bird at 12lbs. most likely needs a lot a ground to get airborne? Much like the real pig...
If you want to go faster, first try increasing the pitch., Tune the engine with new prop and go for a ride. A tuned exhaust system would be very helpful in accomplishing your goals. Weston makes good stuff, check with a dealer for the right application. Mac's Products here in the USA has everything you need. Engine savvy is a absolute requirement when attempting to extract every last bit of power. If you don't own a tachometer, I'd invest in one.
http://www.macspro.com/