My 04 TT is still wearing the original 255/55R18 109V Conti 4x4 Contact All-Seasons at almost 18k with about 4/32" remaining. That's 2/32" off the wear bars so I have to make a change soon. I've scoured the web for info that may help to decide which tire to use as a replacement.
The first choice is whether to stick with all-seasons or go to a summer tire. I was concerned when I went to the Porsche web site and found that the all season tires are no longer offered as an option for the 2005 Turbo with 18" wheels, although it is still available for the S and Cayenne. PCNA said that, because of the rated 180 mph top speed of the Turbo, they could not offer the V speed rated all season tires which are rated for up to 149 mph. That should not be an issue for those of us who can not cruise at sustained speeds in excess of 149 mph. The advantage of using a V rated tire instead of a Y rated is better ride and longer treadlife. Some people have used H rated tires and say they are getting better tread wear but they are sacrificing performance characteristics and the tire strength needed for a nearly 6000 lb vehicle.
I have found that the all season tire has the advantage of better wet road traction, hydroplaning resistance, longer tread wear, and of course, better traction in light snow conditions. Some people complain about poor performance in deep snow but that is to be expected. An all season tire is by no means a snow tire. Many people have been deceived into thinking they should perform just as well and that is far from the truth.
The summer tire has the advantage of better highway stability and better dry traction in cornering, but these advantages only manifest themselves at very high speeds and extreme maneuvers. For the kind of driving that most of us do (even us spirited drivers) the differences in handling will be negligable. The all season tire is supposed to be noisier but I have not found noise to be an issue at all with the Conti's.
It seems that most of the advantages point to continuing with the all seasons. Even the 18k tread life, which I wasn't happy with when I started this thread, I now realize is pretty good for a high performance tire on a very heavy high performance car.
As to which brand to choose, I have learned about the significance of the Porsche N-Specification assigned to some tires. N-Spec'd tires are the result of a joint engineering effort between the tire manufacturer and Porsche. The tires are designed and manufactured to Porsche standards to meet Porsche handling, safety and performance characteristics and that are specific to a particular car. PCNA said that, although a car owner can choose any replacement tire that they want, and there are a number of respectable choices, Porsche owners would be getting the best tire to match their car's performance by staying with the N-Specification tires.
So it looks like I might stick with the Continental 4x4 Contact All-Season. They actually have treated me very well. The Pirelli Scorpion Zero is also a N-Spec'd all season tire, but reviews indicate that tread wear is not so good and it is somewhat noisy. That might be because of it's aggressive tread pattern. Tire Rack has a lot of reviews for the Conti 4x4 Contact and the overall rating shows up a not so good. When you use the filter to look at reviews from Cayenne and Tourag owners, which are the only reviews that are relevent, the overall results are very positive.