none of them lowering springs work for the miata. with the stock springs you have like 40-50mm travel in the back till you sit on the bumpstops. if you put in shorter springs you will have less room for travel. the FM springs are bvlack and totaly look like stock ones!!! if you want something that rides nice and works, go with a legit coilover suspension. trust me, i was riding around on h&R springs with bilstein B6 dampers for about 4 years, and after i installed my D2 coilovers, i recognized how shitty the ride was before!
i would never every waste money on that kind of loweringsprings, i would even go with a raceland coilover set, even they ride much better then mine did with the old setup.