Traction is not a problem in the Prius unless the tires are bald or in wet weather. Are you getting 17s in the near future or doyou want a tire for your 15s? Keep in mind that a full on performance tire will kill your gas mileage.
I really don't see why you need the stickiest tire. The Prius is not a super car and in rare cases does it need anything better than the typical performance tire unless all you are going to do is corner carve and race. Seriously, if you have money to burn you can go that route but MPG and tire wear is going to drain your wallet. Typical ultra-high performance tires designed for racing may only last you 20k-30k miles. I think it's a bad decision to go this route but since you are interested you may check out these tires: Only make sure you stick to the 215/45/17 unless you have a wider wheel that can accommodate the 225/45/17. Bridgestone Potenza RE-11 (Extreme Performance Summer 225/45R17) Yokohama ADVAN Neova AD08 (Extreme Performance Summer 225/45R17) Dunlop Direzza Sport Z1 Star Spec (Extreme Performance Summer 225/45R17)
I run the AD08s, though I might go for the Star Specs when these wear out. The grip is epic - in that way, it rides a little like my Evo IX. However, MPG does suffer by ~5-8mpg. If ultimate grip is your concern, you can get ADVAN A048s (UTQG of 60 ) or some R-Comps. The suspension isn't tuned to take advantage of all that grip though - I'd install some HKS coilovers if I wanted to maximize handling performance.
