Keep using that type of plug, try a new one. I've been running my Reedy at max 240 with great performance. Yeah this engine been ran at higher temps but I think 300 for a race tune is a little much. Try richening your top an hour at a time until your at 3 and see how good top end performance you get. Do a few short bursts of throttle and let it idle. See how long it takes for your idle to come down. If it takes a pretty long time i.e. greater than 15 seconds, your probably a little too lean on the bottom. From the idle position grab the gas hard and tell us the reaction of the engine. lots of smoke? bogs?

Also what type of fuel are you using and are the shims in the engine stock?