Your rear tires will definitely wear out more than the front because your car is rear wheel drive and depending on the driving conditions (sudden accelarations, lots of uphill climbing, etc.) your drive wheels will not last as long as your front wheels, that is why there is a need to constantly rotate your tires so they would all last equally as long. However, 5000km is sure on the short end of tire milage. As I previously posted, lowering a car improperly causes the suspention geometry to change (not just camber) dramaticaly leading to excessive tire wear. I experienced the same problem with my 260E since I lowered it by 2 inches by simply cutting off the springs. The rear tires were worn out more excessively though the wear patern on the tires was more or less even. My mechanic suggested that I change to real lowering springs so I used H&R but I still cut this as the car was still too high for me. The tire wear was less than when I was using chopped stock springs but was still wearing out abnormally. What I did next was use a higher rubber bump on the rear springs as I had the thinnest (1 bump). The rear end was higher than the front as a result of this but this is hardly noticable. However my front and rear tires now wear out at almost the same rate. You might want to try this out first ( getting a higher bump for ypur springs). This will give you the added benefit of being able to check the other rear suspension components as well.
Hope this helps.