The thing is, it's not the straps/heel cup, but the footbed/gas pedal that's the issue.
I've been on the white, glow-in-the-dark Atlases since they came out.
M/L and Van's V-66 sive 10.5 boots.
They work, but the footbed on these doesn't touch the board, it floats/slides on top of the binding base. If you have them out on the 3'rd position, it's hanging way out in air with a 1/2 gap until your board edge. It's just hanging there in space with daylight underneath you toes.
Personally, the L/XL would be the way to go. You might get a little less mass on the M/L, but mechanically it's robbing you of a lot of energy to be on your toe edge.
Union's footbeds on the contacts had less plastic on the base, and more on the footbed making contact with your board deck.
Atlas footbeds also have pretty nil padding underfoot.
Also, why not get the t.rice/asadachi ?