I used os 46 ax with stock muffler on my phoenix extra 300s. The muffler actually pushes hard on the fuse, and makes aminor dent where the muffler pushes fuse. It is a tightfit - yes,but it works ok for me, so I did not use the extension.
One thing I noticed about this plane is that she becomesunstable and shows dangerous signs of bad habit at low speed. She will be just fine and willfly great as long as you maintain reasonable speed, except when you are perfectly level for landing. Enjoy!!!