I haven't put much effort into riding any wider of a bar, but I generally max out at 760mm for being comfortable. It's what I run on my DH bike and AM bike. My hard tail is at 740mm and I have no plans to change that either.
I always think about bar width and sweep when I grab my mountain bike after an extended period of only riding drop bars. The mtb bars always feel a little weird after and I wonder if it's because they might be putting my hands in an unnatural position that could be improved with a different width, rise, or sweep. Then I ride and stop caring.