like keeping the engine at high rpms to long?

No, but that could do it. If you have a significant amount of resistance to flow, you will build more backpressure than the system is designed for. The amount of backpressure you have is dependent on the resistance and the amount of air that the engine is attempting to expell into the exhaust system. Running at 4,000 rpms with the car in Park isn't nearly as much as if you were running 2,000 rpms with your foot to the floor. (more air being drawn in due to Wide Open Throttle) You have to think about backpressure as a function of how many cubic feet per minute you're drawing into the engine (and thus combusting and expelling into the exhaust) and the resistance of the exhaust system.

Have you removed the 13mm nut on the exhaust gas analyzer port (just before the cat). Did this help at all?