I dont think gears are going to do the trick. Heads will be your best bet, especially if you have stock ones. I dont know what heads the 215s are, but I think 781 and 049 castings are the best of the stock heads. There goes your compression tho if i was guessing. 781/049 heads are 119cc.
I was going to say that gears wouldnt help unless your cam was wild enough to have a high rpm power band and if the engine can handle higher RPMs but by the sound of things, theres no way that engine could perform at any higher RPM than what you are running it at now. It would be gasping for air. I tend to think that lower gears might actually make you slower (because it would need higher RPMs and thus need more fuel/air which your heads cant provide)
I say get some good heads.