Flowmaster 60 series would give you a nicer sounding exhaust without being ricey and some nice deeper exhaust notes. Upgrade these 3 if your lights are dimming on you with higher drawing sound systems. Here is the list;
Battery to chassis ground
Engine ground
Alternator to battery wiring
Now adding a higher amp alternator is easy to do you just need to take your stock 1 to an alternator repair depot and they can give you a higher amp rating one by replacing the internals, adding a second 1 is not really necessary unless you are really going for that competition type of deal. I just did a 3000 watt peak install on a cobalt and it runs fine at higher draws with his upgraded battery and grounds. I put an optima in I just took the dimensions of the old battery and got a yellow top that fit those dimensions. Then I upgraded the battery ground and engine ground. With a draw of 1500 watts continuous the lights were still nice and bright (thats 125 amps) I told him that we would leave the battery to alternator wire as his was a newer car and we would see how it behaved like that. Now with that said I have to correct Black_dub, a multimeter will tell you voltage, they will not read amps without an amp clamp and thats what really tells you about the state of the electrical system. Voltage should read about 14.2 with the car running at the battery and at the headlights. If you are getting less than 12 volts at the battery or headlights then you have problems. My charging system on my mx-3 reads 14.2 while at idle once I fixed the grounds and charge wire from the alternator to my battery. Use an amp clamp or ammeter to see what kind of draw is coming off the battery when you are in a high draw scenario for the sound system and try to see where the problem is. At 200 amps almost all car's charging systems are being taxed to the max, thats a pretty high draw and the alternator will get pretty darn hot at that kind of draw. The cobalt does not use a special battery at all they use a tube to vent the gases from the battery to the exterior of the vehicle. Its a standard lead acid battery. A gel cell like an optima does not get hot unless you are putting a very high draw on it as will 99% of all batteries except for a dry cell battery. Gel cells offer superior vibration resistance but do not stand up to fast charging, they need to be slow charged for maximum battery life as do all deep cycle batteries, they just take a lot more charge/discharge cycles than a typical lead acid battery.