I would add a rear bar before upping the size of the front bar. On a car that is already prone to understeer, adding a larger front bar will only induce more understeer. Add a rear swaybar. If you then find it prone to oversteer, play with your damper settings. Usually damper settings alone will allow you to tune for an almost completely neutral suspension with a rear swaybar. If you find damper settings aren't enough, or if you don't have adjustable dampers, look into a larger front bar to balance the rear swarbar. I'm runnin my 22mm stock front bar with a 22mm rear bar, and my handling is almost completely neutral. I can stiffen my rear dampers if I want to tune more oversteer. I can stiffen the fronts if I want to tune more understeer. Right now, my damper settings are equal front to back, and my handling is pretty neutral. Changing you spring rates will also have an effect on handling characteristics. Running stiffer rar spring rates will induce oversteer. Running stiffer front rates will induce understeer. Generally, a car that is neutral with a slight tendency to understeer is the easiest to control.