Those are great if you drive the Cobalt in the summer only. They were very very very fine tuned to specific conditions, too much so for a canned tune to be safe.
Honestly it’s up to you. I left mine with just intake and exhaust, no stage kits, but I had a reason for that. I auto-x and to me modding the S/C pulley and going into street mod would have zapped all the fun right out of it. So I’m going more the lines of suspension/handling as I have no insterest what so ever in straight line speed.
I’m assuming the $850 was installed? If they’re telling you they list for $1k, they’re full of it. The kit is listed from GM as $750 retail. Don’t let them tell you you have to buy the kits separately, if you buy the S2 kit right off the bat then S1 is included in that. If you buy them separate they’re $950. If you want to purchase the kit then just have the dealer install it, you can get the Stage 2 complete kit (including the S1 parts) for $550.