Yes, 20mm is significant.

My question is why would you put a 20mm setback post on a bike with the same geometry as one that fits you with a 0mm setback post? Don't change the stem, change the post. The post is how you set your knee angle. Trying to accomodate for the extra 20mm of saddle setback with a shorter stem is likely just going to put more weight on your hands, and your knees still won't like it.