I have flow bindings so anything is better than my old Burton's with the quick pull strap that wrapped around the highback clamp.
You should be happy with yours too, the whole boot has a slimmer profile compared to the Burton's in the smaller size.
Do you mean the profile of the back side of the boot? I like how ThirtyTwo boots are straight up at the back and the sole is in aligned with the back. That's not the case on some Burton boots like you said, and it's probably worse on the Ride boots I have now- the back of the boot is completely curved inward and the sole is not flush with it. This results in the boot being set more forward in the binding than it should be, and the only contact points being at the bottom of the heel loop and the very top of the highback.
I noticed that Ride's bindings have higher heel loops and the highbacks have the matching curve, but it means that their boots will only fit best with their bindings.
Here's the boot I have now:
Another issue is that the toe section is really wide and flat, which means I can't properly cap my Union toe straps and have to run them over the top.
I see a lot of pros and other people who ride ThirtyTwo boots with Union bindings, and Union/C3 always uses ThirtyTwo boots in their binding setup videos.
Now that I'm discussing boot-binding fit, I'll go on to say that although ThirtyTwo boots and Union bindings were meant to be compatible with any other brand, Union bindings were likely to have been designed specifically around ThirtyTwo boots. That's pretty obvious from all the official content released that show the two brands together, although they're not really affiliated with each other.
Looks like I'm on the way to a perfect fit!