Yes, a leaky muffler has a very bad effect on the GMS47 -- but only because the carby is often defective in the first place.

I've seen otherwise well-behaved GMS47s go to hell in a handbasket as soon as they started leaking oil between the sections. I guess they were also leaking pressure and that meant they would start to sag in verticals and run erratically. Tightening and sealing the muffler fixed the problem and restored the engine to its previous reliable operation.

However, almost every other modern 2-stroke will run fine *without* muffler pressure so this is only one of many factors in the GMS saga.