[burgermeister]

+1 never mix & match, especially on a car with as little understeer as a 911.

The cornering compliance of the 2 tires needs to match (the wider tire with more pressure in it will make the rear less in this case), or the front needs to have larger cornering compliance than the rear. The other way around (rear has more compliance than front) can create an oversteering car.

Tire cornering compliance is a function of tread and sidewall design (and tire width, aspect ratio, pressure, and load)

As tire manufacturers don't publish such data, unless you have a pal in tire & wheel systems at a car manufacturer, your best bet is to keep the same tire front and rear.

Then there is the problem of absolute grip, which may also be different between different tires. The problem is a different one, but with the same end-swapping potential.