How often do you plan to auto-x and do track days?
How much comfort are you willing to give up?
For reference, my friend with a 94 R-Package is running 600# hypercoils springs up front and 375# in the rear with race Konis. But he does track days at least twice a month with sticky rubbers. And its also his daily driver! Let me tell you, its painful.
Another friend that tracks his miata but mostly does autocross is using Koni sport with 525# up front and Koni race in the rear with 300# rates and its smooth as butter.
I'm running 500# F and 300# R with Tein Flex but I'm thinking about upgrading to 2.25ID (less weight!) Hypercoil springs (575#F and 350#R) once the weather gets cooler so I can goto track days/drift with sticky tires.
Here;s more info on the hypercoils,
http://www.tripointengineering.com/information.php?info_id=12 < as you can see, you can get it in any increment and size you want. And if you do more googling, you can find shops that sell it for as little as $45 each, but the usual price is around $50. They also have a lifetime sag guarantee.
Regarding spring rates, the general rule of thumb with miatas is that once you go with stiffer springs, you want to keep a difference in rates between the front and back no smaller then 250#-300#. For example, if you are running 600# front, don't run anything stiffer then 350#-300# in the rear. By nature of its rwd design and 50/50 weight balance, you WANT the rears to be softer then the fronts, unless you got mad dorifto skills.
Another racer trick, get your car on some corner-weights. The weight of each corner should be the maximum rate of your springs. For example, your driver front weighs 600#, if you get a spring rate of 700#, it's overkill, the springs won't even move. The exception with miatas or tail-happy cars is, of course, the rears should be softer.
Another thing, the Koni sports won't be able to handle anything above 575#, you will need the race ones if you want to go stiffer.
I hope thats enough info to get you thinking!
