When we bought our 2002 Carrera we were told to expect 12000 miles from
the rears (Pirelli P Zero Rossos). We replaced the rears at 18,500 and
they could have maybe gone another 1000 before the wear bars were
touched. We don't drive the car hard or race - if you do either 8000
miles is not out of the question.

Try Michelin Pilot Sports or the PS2. Our dealer (where we replaced the
rears) tells us they handle and wear better than Pirellis and are
quieter. Tires that have been approved by Porsche for the cars will have
an "N" designation (N1, N2, etc.) Make sure you use the same "N" front
and rear if you put on "N" tires. If the car is still in warranty you
probably want to use an "N" tire as our dealer said there could be
warranty issues if non "N" tires were put on a car under warranty and
then something happened (like for example a suspension failure that gets
blamed on non Porsche approved tires).

It probably wouldn't hurt to have a four wheel alignment done by a
dealer or other capable Porsche shop. Our Carrera and the your Boxter
are essentially the same forward of the windshield and our fronts are
wearing evenly across the tread face at 19,000 miles........