In a couple of places Susy uses #-hacks to target properties at IE6/7.
I'd argue this is less widely understood than the classic Star Hack, which has the same functional effect.
It also causes problems with CSSO, which in fairness is just as much a bug in CSSO as it is a problem with Susy, but nonetheless I think the prefix could reasonably be a configurable option instead of being hard-coded.
I've submitted a PR with a patch that does this.
I don't see ay reason to add a setting for a hack. If there's a better hack to be using for the same purpose, I'd rather just make the change and move on. There's nothing special about the specific hack.
Thanks, I've committed a fix with hard-coded star-hack in lieu of the setting. It's attached to the original PR.