Tom,
I'm using a Performance Specialties muffler with a 30 deg elbow. The 15x10
shows around 9400 running slightly rich. I've never really leaned it out as it
has more than enough power as it is and has less chance of backfiring and
throwing a prop if it is left slightly rich.
By the way the 4 blade prop was very usable as it had more than enough vertical
power to fly the pattern but there was a noticable difference in going back to the
2 blade.

tommy s