ZIEX 512 vs. ZIEX 912
I have ziex 512s on my wife's car and have recommended them. I find them excellent as a three season summer tire.
Because the company got so much flack for not being good in snow (thats the missing season), they came out with the 912 that is better in snow but not as a good a summer tire.
I find anything that is really good in snow tends to wander especially on roads that have groves worn in them from traffic. (The tires climb in and out of those groves).
Unlike the 912, the 512 doesn't tend to do that. It acts more like a summer tire than a winter one, but the 512 may not be available much longer.
I wouldn't recommend the 912s unless you are concerned about snow.
BTW, Mickey Thompson has come out with some reasonably priced sticky 15s that are no where near as pricey as some of the 15" race tires.
I don't have any experience with them but here is some info.
Search Results for mickey thompson et - SummitRacing.com
Here is a list of sizes for one of their models
Mickey Thompson Performance Tires & Wheels
(see the first 4 lines of that chart for smaller 15" models)