The horn isn't actually what causes rolloff at high frequencies, it is the driver. Some horns provide acoustic EQ from collapsing directivity. Naturally, if directivity is constant, no acoustic EQ is provided so electrical EQ is desired. Some horns provide constant directivity in the horizontal plane but collapsing directivity in the vertical plane, so they'll need less EQ.

Another thing to consider is the shape of the response curve. Most 1" compression drivers have pretty flat response up to about 3kHz - 5kHz. Above that, response falls off at a 6dB/octave slope for an octave or a little more, to the upper cutoff point, where response drops rapidly. The conjugate curve for power response isn't a diagonal line, rather, it starts off with flat response up to 3kHz - 5kHz and then starts to rise.