Aside from the compression part of this issue, you did say your plugs looked really lean. What mods (other than the cool head) are you running with what jets and carb size? When you ran those new plugs, did you run it WOT through 6th gear? It's always good to warm your banshee up on old plugs, slap the new ones in there, fire it up and rip it through 6th gear...pull the clutch, kill the engine, and keep the throttle pinned until you come to a stop. (You may already know this but figured I'd explain in case you didn't )
Keep us posted!