Thats a wicked miata!
To the op: I think it really depends on your driving goals and how many events you are really running a year. If you're going out once a month (or more - lucky dog) and you want to master car control with minimal consumables cost, then the Miata is the way to go. They are awesome cars - but as you said, with stock motors they are "momentum" cars.
If I had another garage space, I'd def have a spec miata or an Atom. The people / instructors I respect most at the track have all reccomended SMs.