you figure the average 100A alt can run 1000w with no issues at all, and 40-50% of its output is absorbed by the car's stock electrical system, it stands to reason 3000-4000 should be doable if your alt is ACTUALLY 250A.
It's all going to depend on the position of the volume knob and the gain settings. I could probably do 10kW rated on that for the simple reason that I won't crank the crap out of the volume. That sounds like about the electrical setup I would like for maybe 1500w, though I would have a second battery as well. Firm believer in overkill. Much better to have more than you need that need more than you have.