If you are installing the PSE as an DIY kit, then simply do not install the speed signal wire to the cutout relay. On earlier cars this was the Pink/grey wire that went from the PSE relay to the speed signal jumper plug on the relay support bracket(this is the thing where all of the relays are under the drivers side of the dash). This allow the ability to use the switch to quiet the exhaust on demand, but will not activate the automatic cutout at speed.

If you can take a picture of the wiring harness, I can show you exactly which wire not to install.

From the picture you have in this thread, it looks like mechanics for the 997 PSE are very similar to the one for the 996.

If you have a copy of the Technical Information bulletin for retrofitting the 997 sport exhaust, there should be a table that describes the wiring assignments for the cable harness. Look to see if there is a wire called "Speedometer A Signal" or something similar. That is the wire that you do not want to hook up. Without the ability to know how fast the car is going, the relay will not activate in the 50-80 kph range.