what up dave? if anything it's gonna cause a lot more back pressure. 2 stroke mufflers are designed to be expansion chambers to allow the exhaust pulse to aid in the combustion process . but it don't take a genius to see that a stock 2stk muffler has a lot more internal volume when compared to a performance pipe like a mac's pipe for instance or for instance a pitt's or bisson style muffler.

probably best to slap a 16x6 apc prop on it and either tie down the tail or mount the engine on a test stand and 'safely' test it before chancing the airframe to an untimely demise. luck, and blue skies brother