Well I do have one FP25 and too many to count FP20s. This is because I never want to run out of FP20s! Fit out an FP20 as per the BBTU as preached by Dan Rutherford and you will have the best possible pipe style run out of the many other small bore engine. Without the BBTU, the FP25 probably wins over the FP20 on purely power output.
Dan Rutherford used to convert FP25s to FP 20s so that should tell you something. I have tried the BBTU on an Fp25 and it just doesn't work anywhere near as well as it does on the FP20.

Regards,

Andrew.

It's really complicated:

Use the E2030 muffler that came with most FP 20s (and comes with the LA 25), use an APC 9-4 prop, and the stock FP-20 venturi.  Convert to a regular needle valve per your preference: the regular needle valve works better, the remote needle valve survives full-power inverted landings better.

If you ask Dan Rutherford very nicely, he will send you his monumental epistle on the FP20 BBTU. It will tell you every last detail of the conversion, plus some things that are not! I like Dan's style of delivery!
Look up Dan Rutherford in the members list and send him an email, that should get you a 12 or 13 page epistle which tells you all!

Question about the setup that I dont recall seeing discussed.   
Is it important to seal the muffler-engine flange as is the case with piped engines ?