I put the horns in both 15/6/1's. I didn't do the last highpass stage on the crossover. Instead, I run the horns off my GK 1001 rb 2's high section. 5K and up, the control is on the amp. The horns can add a nice breathy top end. I think they would be even better if they crossed over at a lower frequency.
I think this was addressed in the latest version of the high pass. The first versions crossed at 6K or something and the horns were pretty subtle. The ASD 1001 goes down to 1K, so the lower crossover point would make the horn more pronounced
My problem is that this set up is dependent on the GK, and now that I'm moving towards a new setup I'll need to get in and add the high pass. No bid deal, it's only a few bucks in parts.
When I use the 12/6 cube without a horn, I really don't miss it at all. If I run the 12/6 and the 15/6/1 with the GK I turn up some horn.
The horns are pretty cheap, like $30 for the ASD1001 and $7 for the wave guide, IIRC, and easy to install. I use a hole saw. You could add a horn later without too much muss and fuss.
Now that I'm using the 15/6/1's more like PA cabs, with a mixer into a Carvin HD 1500 power amp, I can see adding the highpass to run the horns. Or I could start using an active crossover to really dial things in.