NC Sway Bar Recommendations

OEM
Front 21mm tubular
Rear 11mm base, 12mm sport

Supermiata NC MX5 Sway bar kit

Xida with 280/224 Touring springs
Never 32mm front or 16mm rear. The shocks and springs will control roll enough for street and casual auto-x or HPDE. Too much sway bar will make the car skittish and drastically reduce ride quality

For >280tw or tires 225 or smaller, 7-8″ wheels, stick with OEM bars

If 245’s or <280tw:
Racing Beat 25.4 adjustable
RX8 “red” 25.4 (non-adjustable)
RX8 “red/yellow” 25.4 (non-adjustable)
Not RX8 yellow 26.8

Xida with 616/392 Race springs
245 or smaller tire, 280tw or softer, 7~8.5″ wheel:
Racing Beat  25.4 + Whiteline 16mm rear
RX8 orange 26.8 + Whiteline 16mm rear
RX8 yellow 26.8 + Whiteline 16mm rear

255-285 on 9-11″, super200 tire, or any size Hoosier on any size wheel:

RB 32mm front + Whiteline 16mm rear
RX8 bar not big enough

The 19mm rear is really too big for anything. If your NC understeers, something else is wrong. Perhaps more front camber, fresher tires, different spring rates, more rake. Some urethane and delrin bushings kits cause severe binding in the front suspension with cause understeer. Adding a 19mm rear to band-aid understeer caused by a bad set up will just make the car skittish and reduce overall peak grip.