NB Miata: Valve clearances: How tight is too tight?

I've been chasing a P0302 on my NB for a while now. All the obvious things have been replaced/tried.

I tried measuring my valve clearances. The intake valves were within spec, but the exhaust valves were a little tight [.008-.009, I believe]. I'm going to go remeasure the clearances to double check when it's not 900 billion degrees outside (gotta love Florida...)

My question is: How tight is too tight for valve clearances? How tight until they start causing misfires?

I've read threads of people having ZERO clearance and having misfire problems, and while that would definitely be the cause for my issue, I still have clearance on mine.

Edit: Just in case my other threads haven't been read, my symptoms are cold misfires [especially part throttle and idling], P0302, and continues until the engine is hot, then the car runs great and idles great.