When you increase the wheel diameter, you affect two things:
(1) Reduce tire compliance
(2) Typically (through not always) increase unspring weight
Assuming both things happen when you move to a larger diameter rim, it affects the car in the following ways:
(1) Reduced tire compliance affects the suspension tuning -- specifically quicker response time. Most enthuasists would say this is a good thing.
(2) Increased NVH - when I moved from 16" to 17" on my S2, the NVH
dramatically increased.
(3) Increased rate of wear on suspension components because more shock is transmitted from the road to the suspension components -- this is the main reason why Porsche does not recommend fitting 18" or larger on your 968. They worry about things like cracking spindles, bushing failure, etc. Are they overly worried? For you to decide...
(4) Increased unspring weight means your suspension can't control wheel motion as well over bumps. The net results is an overall reduction in grip over bumps.
(5) Reduced tire wall height makes it harder for the suspension geometry to keep the contact patch flat as the suspension travels. This is especially true of MacPherson strut or trailing arm suspensions used in the 968 (front and rear respectively). The result is less overall grip over large ranges of suspension motion when combined with body roll.
Just my 2c...
Karl.
