Unless I missed it somewhere another thing to consider is how it will fit on your head. Padding and / or hardhat suspension liners will take up a little bit of room too. I can fit my head inside an AP bucket with barely enough room for the suspension system. It's a perfect fit for my head.
On the flipside, I've got a suspension system, hoses, and fans in the FX based Magmatrooper helmet and still have room for a duffel-bag. While it looks pretty good from the outside it feels huge on me. I've gotten used to it, but for those who are looking that is something to be aware of.