The reason people do it is to pull the restrictor plate on 1.8L cars. SM directly cross classes into E3, but you must run the restrictor plate. You are allowed to run any DOT tire. Any spec miata prepped to the limit of the SM rules is a PTD car, thus E2 if you want to use PT as its non-enduro declared class. Classed as a PT car your tire choice comes into play...
It does somewhat depend on the year....but this is how it works out. Two things to keep in mind first you cant get points back for running heavier and two this is only potential SM mods, not all cars have all these of course:
1.6L car base PT class is PTF** at 2182lbs
Engine
+1 Intake (Cone filter)
+3 Exhaust (+2 cat back, +1 for cat removal)
+2 Fuel Pressure Regulator
+6 If any of the SM legal "pro motor" head work is done (this one is debated back and forth and probably depends on the regional PT director or tech director's discretion, i am both in my region and i don't apply it simply because it's not enforceable in any realistic way)
Suspension
+2 Sway Bars
+2 Springs
+3 Shocks
Aero
+3 Chin Spoiler
Drivetrain
+3 LSD
Chassis
+2 bump steer mod (if employing one OR both of, rack shims and "R" model tie rod ends)
+3 for chassis bracing (if updated to the 97 sub-frame braces)
Tires (base class tire size is 215)
+8 205 RRs (9pt tires -1 pt for size)
+9 205 SM6s (10pt tires -1 pt for size)
Potential total of 38-39 pts....more than 24 puts it in PTD
1.8L 94-97 car base PT class is PTE at 2330lbs
Engine
+3 Exhaust (+2 cat back, +1 for cat removal)
+2 Fuel Pressure Regulator
+6 If any of the SM legal "pro motor" head work
Suspension
+2 Sway Bars
+2 Springs
+3 Shocks
Aero
+3 Chin Spoiler
Drivetrain
+3 LSD
Chassis
+2 bump steer mod (if employing one OR both of, rack shims and "R" model tie rod ends)
Tires (base class tire size is 235)
+2 205 RRs (9pt tires -7 pt for size)
+3 205 SM6s (10pt tires -7 pt for size)
Potential total of 28-29 pts....more than 19 puts it in PTD
NB 99+ chassis cars base PT class is PTE at 2400lbs
Engine
+3 Exhaust (+2 cat back, +1 for cat removal)
+2 Fuel Pressure Regulator
+6 If any of the SM legal "pro motor" head work
Suspension
+2 Sway Bars
+2 Springs
+3 Shocks
Aero
+3 Chin Spoiler
Drivetrain
+3 LSD
Tires (base class tire size is 235)
+2 205 RRs (9pt tires -7 pt for size)
+3 205 SM6s (10pt tires -7 pt for size)
Potential total of 26-27 pts....more than 19 puts it in PTD
All that being said, if you are heII bent to pull your restrictor for enduros your best bet will be to re-baseclass the car by getting 3 dynojet runs without it and sending them along with your minimum weight to Greg (National PT director). He will reclass the car and you wont have to take any engine or weight points...i believe most cars then can squeeze into PTE sans restrictor plate.
EDIT: Fixed 1.6 max points before jumping to PTD