Based on my experiences doing something similar (on a different vehicle), the 20" winter tires will certainly perform better than the 20" performance rubber, but will not perform nearly as well as winter tires on 17" or 18" wheels. The wide footprint will tend to float rather than dig for traction, and the lack of sidewall flexibility reduces the ability of the sipes and shoulder blocks to do their job. The difference is quite surprising. Regardless, it's way better than doing nothing...