If you overload your little motor on too large a propellor you will lose not only huge amounts of performance (it may not even leave the ground) but you risk quickly burning up the motor. Try finding the correct size of three bladed propellor for your motor. You can turn the same RPM but with a smaller diameter blade the tip speed will be significantly slower resulting in far less noise. Also, go for the good muffler. I think bewteen a good muffler and 3-bladed propellor you will find that the little motor does not generate that much noise. A good 3-blade will give better performance than a two blade spinning far to slowly and your motor will than you also. Remember that a soft-mounted motor will cut your DBs also by a couple and that can be important.

Mark