Keep in mind that the standard for measuring any speaker's SPL is half-space anechoic, so I wouldn't quibble about a horn being quoted at 108dB. I can't think off hand of any speaker being measured in an omnidirectional mode. When it comes to HF horns the radiating plane is often going to be a wavelength through the majority, if not all, of the passband, so the idea of them radiating onmidirectionally doesn't apply in any event.