You are correct on a real world test you would not have a continuous 500 amp pull. The pull would drop when the bass isn't hitting, which in fact would allow the voltage to recover and go back up. So, on the same exact 30 second run with a song which is dynamic the voltage would be higher than 10.8v.
Look at what your competitors that are limited to a single battery are doing. Ask Derrick Haddock what his 3100 was doing on a Sundown NS1.
Is a single D3100 the best option for a 5K system? Not for competiton use its not. You would see voltage drop, which would loose power from the amp, but a single D3100 with a good 250-270 amp alternator and proper wiring will run a 5k system on a 30 second competiton run and keep the voltage high enough to not send the amp into protect. Or check with Rusty at DC Audio, Nick at Incriminator Audio, Jacob at Sundown Audio, or any of the other amp companies. There is a reason why companies that use to recommend other batteries now are recommending XS Power. That reason is because our competitors have proven what XS Power Batteries do in the lanes in real world conditions. I am not saying and never will say there are not other options out there and some really have no reason to run XS Power, but there isn't another option out there that offers what XS Power does. That has been proven over and over again in the lanes. Slamology and SBN was a great example of this. People that use to try to trash us are now running XS Power, because they tried them and saw the difference in voltage and scores.
In the end if you are running something and it works, then why change? Only person that can decide what you should run is you.