If you want really low your not going to like tein, and k-sport probably. Most people will tell you That SSC or Pedders are the best. I don't know if they are since I've never used them. I couldn't tell you about H&R, since I haven't had them on my car.
As for the tein S-Techs, I like them I've had no problems with them after about 1000 miles now. I have 225-45-17's on my car and in the rear I can barely put my finger between the tire and the rear fender well. The front has a bit more gap, but nearly as much as stock. Ride quality is ok, but not great. With that said I haven't bottomed out here in Hampton Roads VA, and the roads here suck.
K-Sport... JUST SUCKS thats all there is to it, thats my opinion from when they were on my car (about 500 Miles). I have been told that they are better when you pair them with upgraded struts, but then again which springs wouldn't benifit from upgraded struts?
Hope this helps you out some with deciding which springs to go with
A progressive spring is a progressive spring is a progressive spring. Everyone is going to have their opinions. Just know that no aftermarket spring is going to ride anywhere near the comfort level of a stock spring, they are not susposed to, they are not designed to, when you go into a corner and hit that pothole and it feels like your stomach just came up through your throat, It is susposed to be that way. Buy Eibach, K-sport, Tien, H&R they are all like this and most other companies also. I sell these and do this for a living, most of the time it is a matter of personal opinion and likeness the of brand. The only progressive spring that is made for the cobalt that do not have these charastics are the Pedders. They are designed differently, they are still progressive but unlike most companies mentioned above that have taken the time to develop a true performence spring, with no coil bind. The only other spring that was made for the Cobalt that had ride comfort was the Eibach pro kit, because they were a linear spring, just like your factory springs. Now from what I understand from last year at SEMA, Eibach hass now also become a cheapscate and their prokit is now a progressive setup.
So which spring did I buy, having access to allmost every brand, one that I do not sell. K-sports, why? Plane and simple the price and I cut a coil anyways so why spend more money and they are no different than any other cheap aftermarket spring, and they all have reverse rake with a 2-2.5. All of the aftermarket companies that make springs for the Cobalts have virtually the same spring and compression rate.