I've got power grips on my winter bike to allow me to wear snow boots when I need to. It took a tiny bit of getting used to the power grips, but they work great with all kinds of shoes. Unlike regular clips and straps you can adjust their tightness just by moving your foot a little because you enter the straps on a diagonal and then straighten your foot to increase tension on the strap.

switching to spd would not be reccomended. I've read that most spd shoes have a metal plate where the cleat attaches that really sucks heat from your feet badly, making them inappropriate for winter use.