My car does have the stock alty which is 120A.When I first got the car I had a XX Goliath 5000 connected to 1 Optima YellowTop. I had 0 dimming.
These cars actually have a computer controlled voltage regulator. Whe volyage dips past certain point like when the bass hits, the regulator instantly compensates by upping the voltage.
The regulator can actually put out up to 16v. and the factory battery location being in the rear helps alot.
Also, the headlights on these cars draw very little current.
I now am using an XXV Maxximus 10,000 and it will be powered by 8 PowerMaster D925s and 2 Kinetik HC2400s for now.
I will probably be getting the first or one of the first PowerMaster alternators for the HHR. it will be a 250A unit that will bolt right in. Should be available in 2 months or so.