[cockerpunk]

anything with more than 50% of the weight over the rear was inherently built with understeer, because understeer is faster. or more correctly, when driven well, a car with built in understeer is MUCH faster. the 911 even moreso than a boxster. the entire point of the 911 is quell the understeer on turn in, and ride that engine out of the corner.

you can add more oversteer bias, very easily. rear toe, bigger rear bar, less front bar, change the spring rates, the rake, etc. adding 5mm of track with spacers would be a very minor effect.

but honestly, what when you notice a balance change after swapping tires, its probably that your rear tires are actually working, and they werent before. if anything your wheel change should have slightly added oversteer. not a big change, but slightly. but when you swap from old tires, to new tires, often the balance will shift to more understeer, because the rear tires are actually working well.