Well, I purchased an Arai Vector 2 in size XL and it does fit into the side cases.
On an unrelated note, I am not sure I get the Arai mystique. The Arai Vector 2 is a decent helmet, but at a premium price point, but compared to the Bell Star I find the Arai lacking in a few key areas, chin strap is too short, no strap cover to prevent chaffing, strap snap is too close to the helmet side making it difficult to snap into place, no chin curtain making the helmet pretty noisy, shield mechanism is clumsy, prone to fogging, eyeport is narrower and it doesn't vent anywhere near as well as the Star. Unfortunately, the Bell Star XL shares the same shell as the 2XL, and since the 2XL won't fit the side cases, it isn't an option.
So, for now I have the Arai, but I don't think my helmet shopping is quite over yet.
Thanks for all of the recommendations, I do appreciate the suggestions.