Oh, and also, not all 15" speakers are alike when it comes to beaming. Yes, beaming occures because of the diameter of the speaker vs. wavelength, so in theory all 15" speakers would start beaming at the same frequency. But in reality there are quite some differences. Mostly because speakers won't behave like a piston anymore on higher frequencies. So if not all of the cone is moving, the diameter argument isn't fully correct. Above 2kHz it's basically only the dustcap radiating any useful sound. So for those frequencies you could use the diameter of the dustcap for your rule of thumb calculations.
This issue is of course still present in multiple loudspeaker per passband arrangements, but won't be the only factor determining the directivity of that passband.