With your setup, probably as you're talking about another 75 to 100 amps draw, from the end battery, not the electrical biven the way you indicated you use the system. As for the output, this is how that works. The ear perceives a doubling of the volume at around 10 db, For that to occur, use this as a guide.
For a 3 db increase, you will need to go from 3500 to 7000 watts.
For another 3 db increase (6 db total) you'll need to go to 14000 watts
For another 3 db (9 db total) you'll need to go to 28000 watts.
A 1500 increase is going to give you roughly 1.2 db increase or about 12% increase in volume. Granted, you can see that this is starting from a fairly substantial starting point, but hopefully it gives you and idea of what it takes to go from where you are now to where, if anywhere, you decide to go.
Any of those jumps to obtain another 3db would likely need an alt upgrade, but a substantial investment in subs, multiple cable runs or very large cable runs and massive battery banks and parallel grounding runs back through the 2nd battery and to the front battery with additional motor/chassis grounding reinforcement.