I don't know any music theory but I always liked Meshuggah's guitar tone. On Chaosphere, the album that New Millenium Cyanide Christ appears, it sounds really fucking heavy. I know you play in a band so I won't pretend to be amateur music theorist, but to me it depends on the song and the band's catalog. It works for Meshuggah because of their style. They only started using 8-strings later in their career.
Iommi got the most out of his tuning, as did a lot of other great guitarists. I think it comes down the the skill of the musicians and their ability to discern what works best for their style. I hate "metalcore" bands that think breakdowns are heavy simply because it's a breakdown.
If you can get through it, here's what I think a breakdown should sound like. Converge are one of the most technically proficient metal bands in the scene today, and Kurt Ballou has an excellent ear for what tuning and arrangement should be. It'll be at the 0:57 mark or so. They also wear their punk influence on their sleeve, so listener beware.
btw it took me like five minutes to write this