Sway Bars

Copied from Miata.net:

1.Eibach/Mazdaspeed : F 25.4mm R 15.88mm (2way adjustable)
2.Cobalt : F 25mm R 17mm (2way adjustable)
3.H&R : F 22mm (2way adjustable) R 16mm (3way adjustable)
4.Hotchkis : F 27mm (3way adjustable) R 21mm (3way adjustable)
5.Autoexe : F 22mm R 14mm (2way adjustable)
6.IL motosport : F 22mm R 15mm
7.RX8 : "sport" F 27mm R 16mm , "non sport" F 25mm R 16mm

MSR: 27.6F, 19R

Hotchkis & MSR front sway-bar is the same Dia. (26.5mm before powder coating) & wall thickness (4.0mm) as the same the old Mazdaspeed RX-8 front / RX-8 09 / orange and newer front bar.
The 04 to 08 RX-8 sport / yellow front is (26.5mm before powder coating) and 3.5mm wall thickness. 

RX8 Yellow dot bars, 27F, 16R ( 26.8, 15.9)
Goodwin Progress FSB 28.5 mm hollow 6.35 wall, 3 hole adjustment

According to Mazdaspeed motorsport
MX-5 cup sway bar kit - part number GRMS-8M-D16
MS-R sway bars
Part No : 0000-04-5316
Part No : 0000-04-5317

RX8 Sway Bars http://forum.miata.net/vb/showpost.php?p=5347595&postcount=6