Seems for all that has been said, no one has answered your question. Since your intended use is a speed plane by all means use a straight tuned pipe. It does make a sizeable difference even with sport engine timing. If the engine is setup with more than 22 degrees blowdown angle The PIPE makes a HUGE difference.

All this talk about differences between Glow and Gas is hogwash!! Exhaust temps and therefore acoustic wave speed are slightly different, but other than that they both operate on the same laws of physics and both can equally gain a lot of power through the use of a tuned pipe.