Of course any horn should be designed for the particular driver. ..and in the case of a BLH that typically includes some measure of baffle-step support. IF however the driver has some lower-mid. gain (typically 150 - 300 Hz), then you have less to worry about (..or you could just go the route of baffle-step compensation).
additionally, lower Qm can lower the effect of the horn/(resonant t-line) on the driver and reduce non-linearity. (..this is a "tail-wagging-the-dog" effect, with the "dog" being the driver.)
Actually, not too much. But a 135 Hz front loaded horn is of course much better. It would work somehow, I need to measure the 8PE21 with and without the H6000 horn. It is not intended for this application anyway, I just wonder if it would work somehow So the answer is it most probably would, but nothing to be excited about.