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.
2012 - 5.2L R8 v10, 6MT
2011 - 4.0L E92 M3, 6MT
2002 - 3.6L 996 Carrera 4S, 6MT
2004 - 2.2L Honda S2000 , 6MT