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You're assuming that the tyre pressures remain the same. With larger tyres pressures can be reduced, increasing the contact patch without squashing the walls too much.Due to the extra weight at the rear of the 911, the wider tyre just changes the shape of the contact patch. The total contact patch will be the same no matter how wide the tyre, but the shape of it will be wider which is better for a performance car as it causes less tyre deformation, and therefore heat. A 255 tyre will have no more rubber on the road than a 305 tyre, but the breakaway characteristics will be different, and so will the heating characteristics.
You're absolutely right. Tyre pressures would come under "other factors" as if you were running a particularly low pressure, a wider tyre would be beneficial as it has the same effect as increasing the weight on the wheel with regards to the size of the contact patch. This won't affect grip directly though. The fact that there is more rubber on the road still won't gain any more grip in itself, but it will change characteristics slightly.You're assuming that the tyre pressures remain the same. With larger tyres pressures can be reduced, increasing the contact patch without squashing the walls too much.