M5: 4288 lbs, 550 bhp, 400 mm six-piston fixed-caliper compound disc brakes, custom Bavarian elastokinematic suspension honed on German race tracks for decades.
P85D: 4936 lbs, 557 bhp, 355 mm four-piston fixed-calliper regular disc brakes, standard suspension setup you find in a Ford Crown Victoria.
It's not a surprise that a heavier car with the same power and significantly worse brakes and suspension handles significantly worse.
But as you say, no-one is buying a Model S to go play on racetracks. Especially since we know it can't do a full lap of the Nürburgring Nordschliefe without overheating and reducing power output.