What 15mm intake? A stock Moby is considered a 14 or 13.5. So in essence you will have a 15.14 carb, essentially a 14mm carb.
Anyway, people on here all the time say they run this carb and a stock exhaust. Personally I tried it once and it didnt work, but Im sure its something minor I over looked.
I run a 70cc motor and 15.15 carb on the 14mm intake. The stock exhaust was really restrictive, it was about 4mph slower than using a de-restricted VLX exhaust. Due to stripped exhaust threads, I now have to run with no gasket, which means less back pressure, and it is running almost as good as with the de-restricted VLX exhaust. But I do not actually recommend running with no gasket, the exhaust wobbles, exhaust leaks around the port, and sound is actually pretty nice but certainly not quiet.