Steve,
The best answer I can give is that 20's will be terrible in the snow. That is not my opinion it is a fact based on the laws of physics. Stick with 18's and snow tires. 20's are too wide and the sidewall is too small to do well in snow. Ask anyone who drives in snow and knows anything about tires.
Tall and skinny are what you want and the taller the sidewall the better.

 

I went with extra set of 18's with Pirelli Ice & Snows. Worked QUITE well this past winter. Looking forward already to this winter.

 

Steve, is your life worth the extra $2,500.00? Do you want to become an expensive snowplow? Truble is absolutely right about the physics involved.

Early last winter, I bought the factory 18" Turbo Wheels with Dunlop M2 Winter Tire Pkg. on sale from Suncoast Porsche in FL for under $2,400.00 (I think you can get a better deal from Sunset Porsche in OR).

The performance of these tires were very impressive while driving through several unplowed roads and through a snow storm (where I counted "8" 18 wheeler trucks in ditches!) in NW WI.

I still say that IMHO this has been the best option I've bought to date for my CS.

 

I'll second that, and here's a motivator...

 

Anybody know what wheel/tires are on that Cayenne?