I took the car on a test drive where I could really open up the stereo. Bass knob on max, radio on max and I tossed on some Bass Mechanik while driving (2,000 rpm) and the lowest voltage I saw was 13.3v. I was beating those subs as much as I could with all the power I have and 13.3v was as low as it would drop. The second "battery", the Ioxus barely came down 10% of its capacity so the alternator is handling the AGM up front and the Ioxus out back while full tilt on the bass.
This means I can move forward with my plan of a larger sub amp and a 6 channel solution for the active front stage... or the JP7.