the rear swaybar is thicker and willincrease oversteer not understeer. The front swaybar is the same. Those of you looking to get a bigger FRoNT swaybar will cause more understeer should you go that route unless you play around with the castor, toe out, and tire comprounds (front vs back). Most North American suspension setups do not opt to go for the stiffer front swaybar to rotate the car. The Japanese however do use this method with the methods I listed. The BSQ mounting kit for the rear ITR bar onto a GSR or LS is very good.
thickness of the rear one on an ls is 13mm, itr is 22mm and 800% stiffer. i have one on my car. flip the existing brackets so raised part is towards middle of the car. then it bolts right in. there's an article somewhere about it... awesome mod. my car is so flat in the corners and so responsive. do it!!
the sizes are the same on all models for the front bar. they usually come beefy because of the extra weight in the front (i.e. engine, drivetrain, people)