On the wheel back spacing, I don't believe 4" is the max I could run with coilovers. I haven't measured, but I believe 4.5 would fit easily. There's a bit of room between the 215s and the spring.
My car can go way lower than it currently is. In fact, I believe I could rest the frame rails right on the ground. Still plenty of adjustment left, even as low as I'm running. I use the race version of the coilovers, which mounts lower than the stock spring seat, with a longer spring to maintain range of motion. That's the one thing that I'm not sure of with the Miata solution, since it uses the stock spring seat, it must be using a very short spring if you can drop it that low (can't view the pics from this computer).
Spring rates: I just have to say this.
Don't over spring it!!! So many people go for high spring rates like 400+, which is fine for a track car but not exactly daily driver material. I run 275F/150R, and coupled with heavy swaybars the car handles like it's on rails, but it was still comfortable enough to drive it from Michigan to DGRRX. I know I'll get some argument on the spring rates, and I don't want to offend anyone, but I had to state my own conclusions here.
If you haven't seen any of my race videos, check the link in my sig. There are some external videos that show how the car handles the curves. Some of the very old stuff is on RB suspension, which I "upgraded" to first. After about a month I outgrew it and knew that I had wasted my money because I wanted much more than that.
I also wanted to mention, if you go with new struts, I've been very happy with the Illuminas. If you're going to run camber plates from Respeed (I don't know about others), then get the "race" version of the Illumina, which is actually intended for rear of an MR2. This is slightly stiffer at the high setting, and the diameter of the shaft is what is needed to fit the camber plate. If you get the standard version, then you have to have the shaft turned down to a smaller diameter.