On a full power burp... figure about 700A worth of draw. Assuming you get the full 240 from your alt, your batts need to pick up ~ 450A of draw. A pair of good sized CA AGM's can handle that and stay above 12v FOR BURPS. Daily use, you'll never have a CONSTANT draw of that much, but it's a good idea to have that much on tap because once you get past your alt, you will be into the batts pretty good. And if your alt can't keep up with the demand, the batts will only go downhill while it's playing.
SuperCaps can really stabilize voltage in an electrical system that's either eufficienct or really close to being sufficient. Anecdotally... I run an 8k daily. I have a 300A alt, a d5100 underhood and (4) banks of caps. I can run a whole song and stay in the 14's if I'm cruising. I don't fool myself into thinking it's a constant 8k that i'm getting though. Maybe 5k RMS after rise and what not. Enough to get an SHD stinky depending on the song.
For your app... I'd want at least (3) g34's or better total. You'll be good.
