RE: all diamond dust post
The mod that paid the most dividends for me with a K&B 6.5 was to cut the piston skirt off on the bypass side. This lightens the piston and unshrouds the intake ports in the case. The piston also had some meat removed inside the pin boss area. I knife edged the rod, also. The engine needed to be balanced afterwards so I did a trial and error balance job by grinding a couple of shallow divots in the counterweight. It took 2 tries to get the engine running smooth. I also ditched the carb, made a venturi and went with a high pressure bladder.
The cowl is worth the effort, I have seen a cowl make the difference between unloading mass rpm and having the plane act like it has a speed brake deployed.