Thats most of your problem right there, get a bigger muffler. That is way too restrictive, the 37 needs to breathe.

To answer your question I ran +- 11 on a shuttle with 550 glass blades and a Heliproz Maverikk muffler and it didnt bog.

The Hatori is the best but there's some others that will do the job. SB6A I think is the part number.