I'm going to go against the grain here and say no Flex.
I have HAs. (They were replaced with SS as i understand). My friend Mike has Flex.
...we both hate his Flex. I mean sure... the car can get lower, and on paper the 2 areas of adjustability are superior, but realistically, day to day mine rides better, stays out of the bumpstops more, is more controllable, and easier to drive, back to back.
I hate his car when I drive it, and want mine back. He drives mine, and wants his car back... with my HAs.
I am said friend (new member, Matt linked me here).
He hates my Flex more than I do. But I think I hate his depowered steering more than he hates my Flex. I *would* rather have HA's than flex, I think, though. It's also worth nothing, though, that my car has poly bushings that his doesn't. I'm not sure what his rates are (neither is he). He may be softer.
Anyway, I've also got Tein Flex on my STI, so I have some experience with their shortcomings.
The first issue is with the adjustable shock length (height separate from preload). Many people misunderstand this feature. Most people think it allows you to lower your car without losing travel. I used to be one of those people. It is true that it allows you to get your car very low and still have shock travel. The flip-side of that coin is that it forces you to have only as much shock travel as there is room for in a car that's extremely lowered. You do not have the option of raising the car to get travel back. It also means that you can't really run softer spring rates in order to improve your ride quality. That said, things are not completely hopeless.
The second problem is that the settings in the instructions that come with the coilovers are complete crap. With the STI, the front was deep into the bumpstops with out-of-box settings. On the miata, the rear was. On the STI, I cut over an inch off of my bumpstops, running only a total of 1/2", in order to keep the car off the bumpstops during normal driving and cornering. I'm on my 3rd setup on the STI (out of box, softer springs with some preload and cut bumpstops, regular springs with further cut bumpstops and only a little preload in front). It still has issues with picking up the rear tires, so I need to play around with tender springs back there. But it's way, way better now than it was with the out of box settings, and overall I'm pretty happy with it.
My current setup on the miata is 9kg front, 6kg rear (out-of-box is 7f/6r), which was Adrenaline Racing's recommended package. I'm almost positive that he recommends this spring package because the out-of-box settings have the car sitting on the bumpstops at static ride height, which of course dramatically increases the spring rate back there. Right now, I have a little preload in the front (don't remember how much), and over 1" of preload in the rear, and I'd be running more if the adjuster hadn't run out of room. All this preload helps keep it from hitting the bumpstop as much, but on the flipside it makes it very easy to unweight the rear, and it unweights very abrubptly. However, I still have yet to cut the bumpstops. The bumpstops are very long, and I'm going to cut about an inch off of them, which will yield 1.5-2" more wheel travel, which should help things immensely, possibly even giving me the option of going back to the 7/6 rates to improve overall ride quality.
The third problem is damping, where ride quality is concerned. There's just something that seems not quite right about them. They certainly don't have adequate damping to control body motion (general ride, putting handling to the side) when the bumpstops come into the picture, which is why it's extremely important to get them off the bumpstops. But even then, they still seem like maybe they're just a little slow to respond, giving you the "Tein Bounce (TM)" as I call it. I don't understand damping well enough to say exactly what the issue is, but I suspect it could be the nature of twin tube struts, which react slower than monotube. So while they look good on a shock dyno plot, it takes more movement to get them to build up the damping force. I'd love to try some JICs (but I think they'd be harsher on the street, and harder to keep off the bumpstops).
So while I have reservations about recommending the Flex, I don't think all is hopeless. But I think if I were doing it again, I *might* go for FM (or M2) tops with Koni shocks, some kind of spring that's not too stiff and doesn't lower too much (probably Ground Control coil-over adapters). Also, there's a vendor at Miata.net that offers some kind of smaller, progressive bumpstop for the Konis that seems like a really good move. The problem is, that alternative just ends up costing as much as Tein Flex, and you can't EDFC with it, and I didn't like the Koni's /that/ much when I had 'em on my NSX. I would also like to try the SS, because I think they will have more travel for a car at sane street-driven ride heights.
EDFC *is* the hotness. Being able to go "oooh, here's my favorite offramp *click*" and get racetrack settings, and click again for your street settings is awesome. And there's three presets, so you can have the third for "ugh, friggin' speedbumps". You can get EDFC for the SS, though (but you might have to pay a bit more for different tops).
And don't ask Eddie at Adrenaline Racing for setup advice. He'll tell you some settings for a nationally competetive CSP autocross car that are completely irrelevant to you. It'll just make you want to bang your head against your desk. DO buy from adrenaline racing if you're going to get the Teins, though, as his prices, customer service, and the support he gives the miata community are great.
-Mike
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