Hi everyone!
SBrown linked my answer to Jeff's original question (that's a pretty cool trick, by the way) so you can click on that for the lowdown. One of the main goals behind the XTD boots is to have the same last parameters and geometry of the regular Ultra & Prime boots. As Ken points out, the main difference in fit has been liner related, but with the 20/21 season that changes and XTD boots use the same liner constructions found in the regular range of boots (that means same fit, same customization potential, same skiing performance).
In regards to BSL differences between Ultra & Prime, the Ultra boots utilize our thinnest wall thickness possible, which allows us to drop the BSLs of the Ultra & Ultra XTDs down to 300 & 302mm respectively. The Prime versions are ever so slightly thicker (and consequently heavier) and it also means we can go up to a more traditional BSL that many retailers & rental operators have been requesting.
And as for my official job title - I am the product manager of alpine & touring boots at Atomic. This means that I manage the complete range of boots at Atomic, from the first idea drawn on a bar napkin all the way up to finished product in a shop. I don't actually design the boots (there is a properly trained designer for that stuff) but I am always present at every step along the way.
Cheers!