Sundown SAE1000D draws 96 amps of current max (I think, it was osmethign around 96 anyway), and puts out a lil bit more than 1000 watts cleanly... I have mine running off of a Deka Intimidator 9A31 in the trunk and I don't drop lower than 13.6 with my 95 amp alt. However, I got that big of a battery because I'm upgrading my highs with a Sundown SAX 100.4 so you'll easily be fine with just a second battery and big 3.
Battery does not create extra power, but if it's charged it can take some of the strain off of your charging system. With AC on full, head lights, and fog lights on, and interior lights on I don't get noticeable dimming, but sometimes my voltage can get down to 13.2 with the car on, with it off I can drop down to 12.2. But that's playing bass heavy music. And thats still safe voltage.
Look into a Sundown SAE 1000D, if u can find one used they are a GREAT deal, efficient, and sound great, and I'm sure u could get a decent price from jacob new if u can't find a used one.