G'day Mate,
I've been running an OS160FX with a Bisson pitts style muffler for years, with no muffler problems at all.
I do know people that have had engine problems, with the 160FX, but they were running either a prop that was too small, like an 18x6, an 18x8 is minimum. I have heard them complain that it won't rev hard enough, the thing is a 160, not a 46, it is flat out at about 10,000RPM, so it should be propped accordingly. It has huge torque, so don't under prop it with a small prop, & expect it to rev at 12000 RPM, it will not do it happily. I run 16x10, 16x14 or 17x10 props, depending on the plane.
They often complain that the muffler does not give enough muffler pressure, & they block off one muffler tube, which is total crap. If the engine won't run properly without muffler pressure, then the engine needs more running in time.
Muffler pressure is only needed to keep fuel up to the engine during aerobatics, & if the engine can't suck enough fuel, without muffler pressure, then the engine has not developed a good piston ring/ liner seal, which makes good compression & crankcase suction, which is what sucks the fuel into the engine.
Another problem I have seen, & experienced myself, is running too rich, or using a plug that is too hot, like an OS "F", use an OS #8, or for 10% nitro & above, an OS A5.