Sorry I didn't see this question earlier. The front negative camber for our 997.1 Turbo maxes out at around -1.1, and that's what I would recommend. On the very first post of the Bilstein thread in my signature there is a diagram that you could print and give it to your tuner: -1.1 front, -1.6 rear. This is a very good basic setting for a street car and AFAIK is identical (anyone pls corrects as needed) to what is the street (more benign) setting of the GT3. I think you will love the change.

As mentioned, the next step if you want to decrease understeer more is to use adjustable sway bar. Soft front, stiff rear. Unless there are specific reasons otherwise, I would probably not mess with the tire width for understeer/oversteer modification.

One very interesting point is that a 4WD car does inherently *understeer,* as making the front wheel a driven wheel increases the slip angle. I understand that more advanced and professional drivers do want a little oversteer, and not necessarily neutral-steer. The 997.1 Turbo does understeer too much, and Porsche did correct this behavior in 997.2: it has stiffer rear spring and that option called Porsche Torque Vectoring (PTV) to deal with this issue. You are on the right track.
Good and informative thread.