You will definitely run faster times with 21’s. Even more so if using Arachnids (lighter, Forged) over Turbines with a staggered set.
I honestly think the 19’s ride worse too. They almost make like a vibration noise. I only had them on a car I bought in Chicago and drove it back to Vegas. That was the longest trip ever BTW. The 19’s not only made a constant humming noise but they ride awful in my opinion. I thought it was the car cause the tires were basically new since he had other 20’s he purchased. Once I got back and put the 21’s on and it was so much better. That’s the only time I ever had 19’s on. To be honest the 21’s with Tesla’s recommended tires have more than enough sidewall to keep the ride smooth and from getting damaged even in Los Angeles. You just might want to put them at 50psi. If anything the tire will get a bubble before the rims gets damaged. The S is a heavy car obviously but I know plenty of people who run 255 30 21 fronts and 295 25 21 rears even though they don’t pass the load requirements with no issues.
Yeah but what TIRES did you run? The choice of tires may affect performance more than the wheel choice.
You can't compare dry summer performance on Pilot Sport 4s in hot heather with the performance of all season 19" tires, for example. I bet with the same tires, you will see better straight line performance with 19" wheels.