I cut a hole and put in draw strings on the foot but I did not remove the zipper. when I did get the zipper fully closed I noticed the bag (golite XL) down would compress around my shoulders and arms making it less effective. When I unzipped it slightly it loosened and worked better. If you remove the zipper you are removing the ability to adjust.
I have had some luck with a double bag system 3/4 bag from the top and 3/4 from the bottom. But that's another story.