I think with Ability damage, getting to -5 is the same as bottoming out at 0 in the D&D paradigm.
Anyway, I'm just noodling with this right now. I've lived with the slightly uncomfortable more traditional stat range, in part because it's so traditional. And it's not exactly like this change, if I do go ahead and make it, which I may or may not, is exactly earth-shattering; I've thought of alternatives like it long before I even adopted m20 at all, and I've seen others noodle with the same concept. However, given the premise and point of m20, the more I think about it, the more I think that if I can make minor modifications that make the design more elegant, then I should.
Of course, it would be even more elegant, if the range was simply 1-8. For that, I'd have to redo all of the DCs for everything, including AC, up by
Hmm... more thought.