I think bars should be at least 680mm for XC use. At least 710mm for Single Speed/trail/all mountain use. It feels awkward at first but once you have wide bars, you'll never want to go back.
You can tell this is a weight weenie forum, 710mm isn't "wide" anymore
I've got 690mm on the SS, 720mm on the hardtail and 760mm on the Pitch, Horses for courses though as the 760's won't fit between the trees at places like Cannock Chase!
I can't see this, you don't want leverage at the bars that only tilts the bike, in fact it would be more stable if you held the bars right by the stem (accepting you wouldn't steer straight) as that way the force back from the bars is more in line with the pedal.
Back ot physics, force x distance, if you leg is pushing down with say 100kgf 10cm from the bikes centerline, you'd need to pull up (or push down on the opposite side, of 50/50 of each) with 30kg 33cm from the centerline to keep it still. Obviously that's an oversimplification as you can swing your body and the bike arround to use momentum to keep it upright, and when your really mashing on the pedals you lean the bike away from the pedal being pushed so that your force acts verticaly down on the tyres/center of the bike.