I've had my P85 for about 6,500 miles with 21" tires. Thanks to Tesla service, I was able to use a brand new P85 loaner equipped with 19" tires for the last two days. I know this topic has come up before, but I just had to write about what a difference it made.
The 21" tires give the car a light, silky, liquid feeling. The grip and sure-footedness at speed and in cornering is quite noticable, but if you're driving your Tesla as you would an ordinary car you'd never miss it.
I find the 19" tires make the car feel heavier and more solid. Not a bad heavy. More of a heightened feeling of solidity without the extra grip on launch or in corners. I don't plan on driving in the snow in my P85 (ever), but if I did, I'd choose the 19".