[Michael9218]

Well, don't know what to tell you. The full floating mounting hardware referenced in bold is the rotor ring to hat mounting system that is on both the race and sport versions. And I seriously doubt you're pulling more lateral G's on your car.

Definitely some confusing descriptions out there. This from the Bimmerworld site on the Trophy kits:
"Where Trophy kits become extra special is in the details: the STR calipers are 20% lighter thanks to extra machining, the rotors have 50% more air airflow with a special mounting system for the rotor hat, and pad knock-back is eliminated by using floating hardware between the hat and disc."

I do know that when I ordered my brake kit from Stoptech through my local shop, we talked to Stoptech tech support and they stated the only difference was the addition of silicone dust boots and anti-rattle clips on the sports and the different pad compounds. Can't see how an anti-rattle clip or the silicone dust boots would cause knock back. Mine don't.

Makes me think your issue is with the rotor to hat interface. It's the floating of the rotors that is what prevents the rotor from flexing with the hub. You shouldn't need springs. Piston springs don't prevent knock back, they react to knock back. They are available as a bandaid for the inferior street rotors that cause knock back. If you paid north of $3,000 for the front kit, than it should have the gold colored rotor hats and should definitely not have knock back.

I would pull off your calipers and inspect the rotor to hat mounting pins. My suspicion would be that they may have seized.