Re: carb as a muffler?
The sweet spot for back pressure is zero.
I do not know where the myth came from in thinking you need some. Look at any race machine, unless the track requires them for noise control, you will see no muffler of any kind.
To the OP- restriction in the exhaust WILL make the engine run quieter, along with no power, overheating and burned exhaust valves.
A muffler or 2, and long pipes will quiet engines very well. Those cigar shaped Briggs muffs work pretty good too- real cheap.