Crankpin,
Wow...seeing your pic of your Vertigo fuselage gave me a wave of nostalgia. I flew my Vertigo for four years. The first engine was a Webra Speed .61 and with that engine the Vertigo was just "average". Steve Helms advised me to "get a Rossi".
I did...and he was right. Switching to the piped Rossi made a world of difference. Flying in cool "heavy" air, I could start knife-edge at 20 feet and it would climb-out and do a 400 ft. half-loop or 'Immelman' in knife-edge and probably would have done a full knife-edge loop....but I never had the courage to do the last half of the loop. The axial maneuvers were great at high-speed. So far as which muffler....I would go with a tuned-pipe.
Regards
JC