I've seen a couple 4 door cobalts and g5's with the SS spoiler. I know it's possible and I believe that Dustin is correct in saying that the base of the spoiler will over the old holes. I would suggest first taking some measurements from the SS spoiler and just double checking that you will be good.
Also, to fully seal your trunk I would suggest getting a small piece of metal (aluminum or stainless steel to prevent rusting) and some silicone and sealing it off. Also, you would have to drill holes into your trunk, so getting some rustproofing onto where you drill would be important (buy it from anywhere that sells spray paint or car parts). Easy and cleanest way would to be use a q-tip soaked in the rustproofing and put a couple of coats.
When you install the spoiler use double sided tape to cover pretty much the entire base of the spoiler and silicone sealant around the bolt/stud that's on the spoiler. I also used extra silicone around the outside of the base but cleaned any excess as it seeped out when it was tightened to the trunk.