I went to 800mm wide bars early, back in 2008 when the only option was the FSA Gravity Light Super Wide. I was getting my bike serviced at North Shore Bike Shop and Matt noticed that my controls were right up against my grips and the outside of my grips were word because my hands were hanging off. He suggested that I try these wide bars that he had on order. Haven't really gone much narrower since.
I'd be willing to try something narrower again, maybe in between the 740s I'd been running in early 2008 and the 780mm-800mm that I've been running since.