RE: Sundowner 50
O.P. :
Run an APC 10X9 with a tuned red Jet muffler or tuned pipe. Id stay away from 10X8, 30% and cooler plug on the 55, it will make more power, but it takes a toll on engine life. Expensive fuel consumption goes up, flight time goes down, just not worth it unless you are racing.
Even so the bearings will go south. When the get "gritty" replace them with the stainless bearing kit from only from RC Bearings. Their rear bearing is the only one on the market that's stainless + plastic retainer and it makes a difference in my opinion. I believe the front bearing in the kit is steel, but front's never seem to go bad, so go ahead and use it with confidence.