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FWIW I cautiously avoid the word "upgrade" when choosing sway bars. We change sways because we wish to change the dynamics of a car in very specific ways. If you are running R-comp tires you are generating a lot more lateral G forces and stiffer sways give you more control over your contact patch. If your car is running square you may develop too much oversteer and a stiffer front sway or softer rear may tame that a bit. They are suspension tuning devices, not necessarily an upgrade to every street car.

If you are currently on stock suspension running street tires, I can say that an M030 rear sway will reduce understeer and allow the car to rotate more easily without becoming too tail happy. Figure out where you are in terms of suspension, exact tire compound and sizes, alignment specs, and where you want to go, then very experienced folks can help you make a wise sway bar choice.