I'm not sure why so many people think that stuff sacks are waterproof. Dump one into the bathtub for a minute and see what happens! Even if the fabric is waterproof and the seams are sealed, the drawstring closure lets all kinds of water in. A dry bag closure is needed to keep the water out.
I'm not sure how you would seal the edges of a polycro ground sheet to make a watertight container. If you could, it would be useless as a ground sheet. A pack liner is an enclosed watertight bag, not a sheet of plastic. Either trash compactor bags (if you can find any these days that are unscented--I haven't) or cut down contractor trash bags, both 2 mil plastic, make great pack liners. Twist the liner closed with a candy cane closure.
As others here have said, use either a pack liner or dry bags, not both. As long as you check occasionally for pin holes and patch them, one or the other is enough. Forget the stuff sacks. I use one small silnylon one to organize small stuff (don't want to be digging in the bottom of my pack in the dark for my toothbrush), and that's all.