"Seamless", that's the all important description that is the ultimate goal.
Those Sistines look amazing! And yes, they're overkill for ATMOS - at least at first glance. But to achieve a seamless, or as I call it - cohesive system, better is better. I'm using little 4i's for the four Tops, and IW's for the Surrounds and Rears, and the "system" works great, but there's a lot of room for improvement. 1/2 of 1/2 of 1/2 to infinity means there will never be an end to possible improvement.
When I tweak the system and do a sound check, my go-to movie is Bladerunner. The soundtrack is so immersive. That movie sounds great on my system, and I'm very happy with the result in a multi-function room, and, I cannot install in-ceiling speakers because it's the roof and it's only built with 2x8's so there's not enough depth for speakers plus insulation.
"As Atmos and surround channels, they would not be asked to do anything quite so demanding."
Not quite so demanding most of the time. In a 5.1.x system, the Surrounds will get a lot of activity. When Rears are added, that activity is spread out into the extra speakers. Same goes for ATMOS. When ATMOS speakers are added the sound level from any individual speaker is less than before. When I listen to only the ATMOS there's not a lot of volume and not a lot of activity, MOST of the time, because most of the time it's added ambience, but when that helicopter flies over, or that spaceship (that should not be able to make a sound in space) rumbles overhead, those ATMOS speakers get a workout! So yes, better is better. The Sistine and its 8" woofers will never feel any strain!!
Probably the best advice to give in a situation where there's a choice to be made between one speaker that's obviously qualified, and a step down that's still a fine choice: If you choose the step down, you may always wonder - What if? I had that decision to make less than two years ago and it was between the 11A and 13A. I could afford the 11A and had been drooling over it for many months. I actually completed the purchase - and without listening to the 13A on purpose because I didn't want to spend the extra money. But, before leaving the store, curiosity forced me to listen while still holding the paid invoice in my hand, and 5 seconds into the demo I laughed, shook my head, and told the salesman to sell me the 13A. That is one of the best audio decisions I've made.