I've owned a 50 GT for about 25 years. I ran it stock with 11 x 6 props in .25 to .40 sized sport planes and also ran it stock with 9 x 8 props on a 32 inch delta Diamond Dust model. It was always a crowd pleaser.
Finally it needed a new set of internal parts and I set the exhaust timing close to 180 degrees, opened up the intake and propped it with 7.4 x 8 APC pylon props and Macs pipe and a home made "open chamber" head button. It responded with 170+ mph runs without even getting hot. The typically small .40 sized crankshaft is what limits it's potential to run any stronger..in other words to move up to the Nelson .40 level of power.
Back in the day, Webra ranked very high on the list of mass produced engines for power to weight ratio. The high price was offset with many years of rock solid performance. The high price only hurts for the first couple hours until you get a chance to fly it and see how great it is.