I don't agree with using it in the salt, that's bad advice that will make it worse just as you can see on the GTS above. If it's laid up over winter make sure it gets stored with a warm engine, the water produced in the exhaust sits in the valve on a cold start move off the drive type scenario. Don't start and turn off cold during lay up either.
Get plenty lubricant on the mechanism and get under the bumper and move them every now and then with your hand manually. Easy peasy.
If you do use the car run it with it off for a while too as then the mechanism is used in the closed position as it's open by default.
Ref the warranty Porsche replace everything under warranty that isn't working but doesn't mean it's necessary, these mechanisms are serviceable and repairable and you shouldn't feel obliged to get one because of this issue alone.