Apparently Optima now makes a yellow top that fits the miata and a few people seem to be having good luck with them. But it looks as though they just started producing them the beginning of thise year.
Either way, I'm still on the original 1990 battery, so I could care less at this point. But up unitl the search I just did, I always heard to stay away from Optimas for miatas.
I used to hear great things about redtops (before I got the miata) and in fact my brother had one in a few of his cars way back in the day, but the last year or so I've been hearing more and more negatives. Maybe they had a bad run?