All else fails, pick up a metz 76mz handle mount flash... GN is 76m at 100... Compared to the 58 for the 580 or even 60 for the 600 Canons, it is just more powerful.
That's some powerful flash. Looks like it's going to run >$1100 with the Canon adapter and needed accessories though. It might be cheaper, although more painful, to drag along one of my Alien Bees next time..
It is a beast of a flash... Prices have gone up (surprise surprise) since I last bought on many moons ago. For that price you can pick up a decent strobe kit (alien bee's or white lightings or a cheap profoto) but then you have to worry about stands, awkward transport from display to display... There are trade-offs either way. The metz semi attaches to your camera via bracket and it's a portable easy set up with ettl, strobo/multi flash, modeling lamp, etc... all the functionality we love from our canon flashes but the power of a small strobe. I shot with those back in the film days but when I switched to the 10d, that model couldn't support digital cameras, I moved to a cheaper metz shoemount flash, and then when i got the 7d, moved to canon flashes for the wireless flash triggering... I miss the raw power of this flash but admittedly it's gone up in price and I'd rather grab even more power with white lightnings for half the cost... But then there are the trade off's mentioned before.