The stock sub isn’t inherently bad. It’s looks a lot like a hifi sub (paper, low xmax, small magnet and voice coil). I bet it’s really high efficiency, and sounds good up to a point, which is where the stock dsp will roll it off.
Without changing the dsp, an aftermarket sub would be treated the same way, so I doubt much difference in the envelope. Like @Ratchey said, probably worse as the efficiency will almost certainly be worse on a bumpin sub.
It all comes back really to dealing with the dsp, either bypassing or replacing the factory dsp.
