I presume that the tires are wearing uniformly across the face of the tread, right?
Remember that careless downshifting also slips the rear tires and accelerates wear.
You can get tires with longer treadlife, but for the tires that are sold for that car tend to have soft rubber and correspondingly short life span. Look at the treadwear rating on the tires you consider. You may even have to bump down a speed rating to get into significantly better treadwear. You may have to hand-carry your wheels in to get the tires mounted if you want to buy tires with a lower speed rating, as the shops won't mount them on your car (liability).
I use Michelin X tires on my 328. They wear like iron and they have soft sidewalls so they tolerate the positive camber on my rear wheels without chewing the inside edge of the tire, but they are a lower speed rating so I have to carry the rims in to Sams club to get the tires mounted.