[deliriousga]

That's what I did. I put bi-directional tires on the rear that are a harder compound. They still have AA traction, A heat, 410 wear ratings and a W speed rating. If you're not tracking, and it's not modified, you can't reach the 168mph limit.

Make sure you have your tires balanced often on the Boxster (every 3K miles was recommended by my tire dealer). Since they can't be rotated they have a higher tendency to cup or feather making a lot of noise. Most places have free lifetime balancing with the tires now. Also, try flipping your rear tires side-to-side if you get bi-directional tread. It will help with uneven wear too.

Btw, don't expect perfect silence back there. It's nearly impossible as the tires wear. The balancing will just help keep it down.