I've recently gone from 630 to 750 on my racey hard tail. Shrank the stem by 20 mm at the same time. Barely notice any difference TBH. I'm good at picking lines and ride pretty light (I'm nearly 10 kilos over the weight limit on my 4 year old lightweight race wheels, they done lots and lots of rocky, rooty, high speed descending, with very soft tyres, and haven't been touched!) so this extra leverage they all bang on about hasn't been needed, it just makes the steering slightly lighter when I'm doing the same stuff, if you see what I mean.
Going too wide has given me some laughs, a mate bought some 850 ish risers for his trail bike, and on the twisties he can only effectively reach one grip when he gets to the tightest rolling turns...... He's about 5 foot 3.
Most of my mtbs are between about 730 and 780. Any wider and they start getting harder to control.