freakin fast pretty much summed it up...

The OS manual and prop recommendations are for "ultra-quiet" - not for power.

My experience is the engine struggles a bit with a 13x5 or 13x6 prop (same prop I run on the OS70FS and my old ASP80 FS)

The OS55 engine runs well propped to turn any prop above 11,000 rpm. So yes you can use the larger props. However... it develops its best power as it approaches 15,000 rpm .... so prop selection that delivers around 13,000 to 14,000 peak rpm on the ground will permit the engine to perform at its very best in flight.

It turns out that a 10x7, 10x8, 11x6 and potentially a 11x7 works very well to put the engine into that effective rpm range (regardless of a stock or aftermarket exhaust system). You can probably throw a 12x6 in there for slower flying aircraft too.