'Toothed Belt Service' warning...

I would buy the tool and check the belt tension before I drove that car. Porken's work is outstanding, but that belt is still that belt, and it might have stretched for some reason. Checking it costs nothing; Rebuilding your heads and possibly replacing some pistons can be THOUSANDS of dollars. Every 928 owner should have the belt tension tool. I keep mine in the rear glove compartment at ALL times.

As already said the wire for the warning system is shorting from ground for some reason (come loose) causing your light to come on.
Waste of time checking the tension with a Kempf tool as the tension is already correct with the PKensioner.
A quick visual ispection with a flashlight down the snorkel hole to check belt condition and roughly central position on the cam gear will suffice.
Find the warning sytem wire and check where it attaches to the engine.