I would say that if you have flat bars now, move the grips in to about 405mm outside-to-outside and see how they feel. I would say 405mm sounds too narrow to me. I currently run some Casey's Crazy Bars. The bullhorns are about 400mm apart, and I sometimes use the horns, but my main hand position is outside the horns. Outside width on those is about 666mm, which seemed too wide to me, so after riding that way for a week or so, I pushed my grips, brake levers, and shifter in as far as they could go (the horns being the stopping point) and rode like that until I was sure it was comfortable. then I sawed off the excess bar, I think between 1 and 2 cm on either side. That's still a width of over 600mm, though. It's a pretty personal thing, so it's going to come down to what you're comfortable with.