These cars, as is the case with almost every factory built car, has slight understeer intentionally built in. An alignment can help. More negative camber will reduce understeer but of course running this all the time will affect tire wear also.
You can also reduce understeer with antiroll bars, shock/strut changes,tire widths, tire pressures,ect. Your driving style may also be a factor. Are you on the throttle hard when the understeer occurs? If so,some of the problem is that you've transfered weight off the front tires thereby reducing their traction
The GT has a slightly stiffer
suspension and is more tuned for high performance driving than the V6. Is your rental a "pony package" V6? If not, then it came without a rear antiroll bar which makes the understeer even worse. Mostly due to the tires, the GT also has a higher limit so getting in a GT may cure your problem just by raising the car's capabilities above what you're asking of it.
The real question, though, is not how to reduce your problem. The real question is; Are you routinely pushing the car close enough to it's limit to experience understeer on the street? That's just plain scary. I suggest you first find a racetrack and join the track club. Otherwise, the easiest way to reduce understeer is to slow down.
