nice use of words *impedance rise.... saying box might ruffle some featherscommon impedance rise if roughly 2x what its wired to.so if its wired to 1ohm look at the 2ohm power. probably around 3k
if its wired to .5ohm you are looking at getting the full 4500w out maybe a little more.
not that its going to be mind blowing but power is power and it does sound like you have stout electrical.