Sclippy96:The Tein Flex ship with 390/336 rates from the factory for some odd reason, I guess they want to make the car feel real sporty in everyday driving. Instead of going lower on the back I would go higher on the front, it won't make the ride as harsh as you would think. I currently have Tein MonoFlex at 550/336 with big RB front bar and no rear bar with similar alignment to you, and it is way cushier than my friends 700/400 koni/GC setup. My car seems to be well balanced now, I started at 550/390 and it was a bit loose for my driving abilities. The stock valving of the Flex can handle the higher rate springs well, even better than the Monoflex can. I acquired my extra Tein springs from Emilio at 949racing.com, they also have a nice Tein spring chart on his site that shows all the part numbers and sizes available.
+1. You definitely need to go up on front spring rate, not down on the rear. The car is too soft already. The stiffer front springs are not going to affect ride like you think they will.
90 Miata - STS #90/190
If it aint broke, don't fix it.