The last time I had this happen to me, it turned out that after all my high-tech trickery, I had neglected to check the piston ring gap. Embarrassing but true. A new ring from Frank Bowman brought the engine right into the power I had expected. Ring gap should be approximately .003". (That's a really tiny gap!!)

I'm assuming you've provided plently of exhaust exit to compliment your other changes that allow the engine to breathe better. I completely gut the stock muffler, then add two 1/2" i.d. exit tubes.

As a reference, my highly modified Echo 23.6cc turns an APC 16 x 8 prop at 9000, and my souped up Homelite 30cc turns the same prop at 9400 or so. My modified Homelite 25cc turns that prop at about 8500.

Good luck,
AV8TOR