Reduce your drag racing in a straight line and spend more time cornering aggressively and you will have more even wear and longer lasting tires.
As NewArt suggested, have the front alignment checked. The front tires are set from the factory with slight negative camber (static) to improve dynamic handling (when the suspension is compressed during cornering, the wheel will straighten up and be flat to the road surface to maximize grip rather than roll onto the outer portion of the tire if there was no negative camber (which would be reduced grip). The static negative camber setting from the factory can wear the insides of the tires more than the outer portions if you drive mostly highway miles or rarely corner aggressively. This situation can be exacerbated if the previous owner had the alignment set to maximize negative camber in order to maximize cornering ability. An alignment shop can reduce the amount of negative camber to provide more even tire wear at the expense of cornering grip.
8000 Km/5000 miles from a set of rear tires is an indication that you like to use the throttle. This number of miles is fairly normal for a set of rear tires on a sports car.