They were lucky the damage was in the box only. I can understand the derating options. Normal derating the 800-600-800 would apply as you remove tabs on both sides on the center dimmer and only the inner tabs one on the side dimmers. Given this spec on C4's website though, they make it seem like as soon as you have 3 dimmers in a box, regardless of tabs or heatsinks, it is 600-600-600.
Good question for C4.
[Link: control4.com]