NC, 70K+ miles on stock sport suspension.
Aug 2013, precision alignment. More predictable tracking and handling.
Nov 2013, RB sways. Biggest single difference in handling. Flat and stuck to the road, I would give up on a corner before the car would. Some increase in side-to-side stiffness (hit a pothole and you notice it more).
Jan 2015, Progress springs, Koni adjustable shocks, FCM sport for lower suspension, RB end links.
I find it really hard to compare suspensions when it comes to ride. I tried driving over the same roads, "planting" the old suspension in my memory and then comparing it to the new, but it just doesn't work. Now, if I could flip a switch and go from old to new to old to new
It seems to me that the ride is generally more firm but still not harsh. It also seems like the suspension is more compliant on the small stuff, i.e. relatively smooth freeways seem even more smooth on the new suspension. Front dive and rear squat is less, though it wasn't a problem with the stock suspension. My wife says she doesn't really notice a ride difference (she did with the upgraded sway bars).
Handling is much improved. The sway bars made the car corner amazingly well but it would still sidestep in corners when I hit bumps, the springs and shocks fix that (at least as hard as I have pushed it so far). So springs and shocks have made the suspension behave much better, it's more "tuned" to the car.
I replaced the front end links because the stock links were looking pretty worn ... might as well put in some adjustables. I will say that it seems strange that the adjustables, when shortened as much as possible, are about the same length as stock. Seems like these are marketed to those who are lowering their suspension, so shouldn't they be shorter than stock when shortened all the way?
Lowering. It may settle a bit more with miles but I am at 13.5 front and 14 rear; more than enough for me as I wasn't truly interested in lowering the car anyway. I used the jack method to preload my bushings and that worked very well, i.e. with the car on the jack stands I used the floor jack under each spindle and jacked that corner up until it lifted off the jack stands (about as much as the suspension would compress on the ground) and tightened things up. While the car was only lowered about an inch I do notice some differences; I scrape the front on driveway aprons more often and the car is a little harder to get out of (I'm 60, you may not notice it much). Definitely watch out for the concrete stops in parking spots!
Bump stops. I agonized over this more than any single aspect of the upgrade; stay with stock, trim the stock stops, or go with FCM. Lots of good advice on all sides. In the end I went with Brian's advice (he has seen and ridden on more suspensions than most!) and used FCM sport for lowered suspensions. I went with these because I wanted to gain back some of the loss in suspension travel caused by lowering the car (the reason I really wasn't interested in lowering the car). Again, how do you describe the difference? I have never driven a car with this suspension on any other bump stops so I really can't compare (I know, that doesn't help you make your decision!). If I could see under the shock boots I would measure the available travel, I tried but gave up.
Not a hard upgrade to install, the most time consuming aspect was the spring compressor that I borrowed from Autozone; it works fine (especially for the price) but requires a LOT of cranking.
I had the alignment redone and I picked a different shop this time. The first shop had an all mechanical setup and complained every time I asked them for the "before numbers", even though I told them I wanted those when I made the appointment and again when they put the car in the bay. And because it was mechanical I never felt entirely comfortable with the precision of the alignment ... but the car did handle fine. This time I went to a place that has a computer setup and has done many Miatas, if you live in the San Diego area I highly recommend Lutz Tires and Auto Repair. When you go to Lutz ask for John (others there might be as competent but I can vouch for John), he knows his stuff and I didn't even have to ask to sit in the car during the alignment, he just brought over the ladder and held it for me. We set it up with -1.5 camber on all four (-1.5 was the least negative he could get on the back), I might have gone a little more negative on the front but I'm concerned about tire wear. We went with slight toe in on front and back and caster is 5.8. This is a DD, no track, no AutoX; I like having fun on the twisty stuff but there is definitely more freeway in this cars life.
06 Sport, 6MT, Winning Blue / Progress, Koni, FCM, RB sways, RB end links, RPF1 7.5, PSS 215