Since starting this thread I have learned a few more things.
As far as voiding the warranty by changing rim size, that may be possible depending on your choice of rims but for the most part i believe itès Ok and has even been done by the dealership in some cases.
There appears to be a valid argument, besides price, for down sizing your rims for winter tires. By down sizing the rim and using a compensating sized tire you increase the height of the sidewall and get a bit more resiliency when hitting the potholes.