My ’88-911 was overfilled by 5 extra quarts of oil above the top line by an inexperienced mechanic

That can happen to anyone, especially if two people worked on the car at the same time. Drain out enough oil to get it back to the "Max" line, and keep on driving.

The worst thing that could happen, especially if you did a lot of high-speed driving, is the oil would get air whipped into it be being hit by the spinning crank shaft. Hydraulic lifters might get air in them and let the valve train rattle but oil will still be getting to the critical points. Excessive oil could get sprayed onto the cylinder walls and overload the oil control rings. That would lead to blue oil smoke from the exhaust. Those problems will clear up once the level is corrected.

Of bigger concern is did two different people each think they were filling it with new oil, or did one person fill the new oil and forget to drain the old oil? It's standard procedure to make a final level check before the car goes out and obviously that wasn't done or they would have caught it. Return to the shop that did the work and give them the chance to correct their mistake. In the unlikely event something more serious does happen, you want the problem documented on the repair order.