What is your present exhaust port timing on your best setup? I have found raising the timing from the often found 120 to 130 degree range up to 150 degrees to be very beneficial. Also, I would still like to see a comparison of going to a Frank Bowman ring. I think you will find some power in that as well.

It takes a little power to drive the mag, both to generate the current and to move the air with the mag fins. So you will find a bit of power there as well if/when you go to CD ignition.

My Ryobi with the base gasket left out, exhaust ported to 150 degrees, Frank Bowman ring, 11mm carb, aftermarket muffler, and electronic ignition (no magneto) turns an 18 x 6 prop at 8500 rpms. (I know this is too high for the Ryobi and will be putting a larger prop on when I get a chance.)

The 7500 +/- rpms you are getting with an 18 x 8 prop is not bad at all, but there is still some more power to be had.

Good luck and keep going,
AV8TOR