You could probably buy a pair of new 18" wheels and snow tires for around the same price as those 20" snow tires. This will also save you the hassle and cost of having to switch the tires on and off the wheels between seasons. Narrower snow tires will run better in the snow too.
I got a set of 17" wheels and snow tires for $850 out the door from discounttiredirect. I've got a MKS though. You'll probably have to go 18's with the SHO.