For perspective, I had a pair of his ES crossovers in my Khorns (when I had Khorns) and I still have a pair of his ES crossovers in my LaScalas.
I was once at the factory in Hope with a handful of people. (one of the get-togethers that have been done)
One thing we did was Roy took us into his listening room and we did some non-critical listening of different slope crossovers. (along with different drivers and some other things)
My take away was the side by side finally made me understand what people mean when they say "it was like a veil was lifted from the sound" as that is the only way I could think to describe it. Now, could I walk into your room and say "AH HA!!! You ARE (or are not) using ES crossovers? Nope. Not me & my ears. However, when doing them side by side it is clearly noticeable having the extra clarity. Now, is it worth the cost for that bit of clarity? That's a personal call for someone.
I think if you lay into the volume with them, the better isolation of each driver would allow them to play a bit cleaner at higher levels (my opinion, I don't know that factually)
I've sometimes felt that if someone listens to their music as more background music, then a lower order crossover might be more appropriate and if they are a crank it up dude, then the ES's might be a better fit for them.