You are very correct about the toe setting affecting tire wear. From my memory those GTO's have outstanding handling characteristics. 20k is acceptable for that type of car. I know that you sell tires, but the 400 tread rating is a rating given to each tire by the manufacturer of each tire and I have yet to see a high performance tire meet their treadwear rating. You have to take into account th epower being transferred to the rear wheels causing alot of tire squirm, which does happen in normal LOW power passenger cars like cavaliers or taurus's. I have owned several 94 - 99 Trans ams and an eagle talon turbo with a 4cyl turbo all had low profile tires and none made more than 20k. They also use different materials in these tires to keep you safe at 140 miles an hour. And these material don't last long. I never think about tire wear when I purchase a performance car, I worry about the COPS!!!!!!!
Its hard to find a good alignment guy. We had one that could align a car to prevent tire wear not just to specs. I know what you are saying. Enjoy your GTO!!!!!!!!!!!
I worked for NTW now NTB for 4 years in college ( National tire warehouse)