Ideally you want to have the same spec pad on the front and back. From how it was explained to me, most cars from factory come with a slight additional bias towards front pads being used over the rear in terms of the 'best braking performance' (so factory the bias will use an additional 10% force at the front because regular joe deals with it better as less squirm on hard braking).
If you keep OEM on the back and say, CL5+ on the front you're having to get those nice expensive track pads to work even harder as the rear OEMs will give up, so your wear rate will increase on the fronts and you will decrease braking performance % further - of course upgraded fronts will still stop you much better, but not as best as possible. Ideally you want the same spec pads on the front and back and a bias valve to increase the back bias by an additional 10%.
Take what I've said with a pinch of salt though
I don't know motorsport science, it will be different from car to car and it depends how the factory set up the car. But that's what I go on. I've gone for OEM on the rear as although the DS2500's are good on the front, I don't feel they are that much better than OEM front pads. But everyone keeps praising the CL5+'s so I will be getting the full set in a few months time.
Cheers
Ry
