RC, your son has come to realize what many car fanatics don't until they're old and grey -- most modern high-end cars get used the same way aside from the throttle and reach the same speeds outside of a straight line.

As I pointed out elsewhere, look at the average speed distance on the NBR for a 911 Turbo S and a Panamera Turbo S. The difference in average speed is probably not much more than the inaccuracy of the speedometer itself. Can your really tell the difference between 70 mph and 68 mph?

In the real world, you don't wear tires down after 30 minutes, you don't boil your fluids... Both cars will probably be capable of reaching the same speeds you'd be comfortable with on any given road.

After all, Porsche did 7:38 on the NBR in the PTTS. Can anyone on here do 7:38 even in a 911 Turbo S? I'm guess maybe 1 or 2 people tops, and 0.0000001% of Porsche owners globally. 

The reason most people would buy a sports car over a sedan is because they flat out look better and are cooler. In the case of the Panamera, it's flat out unattractive.