IMO, it is very rare that a flash in auto mode is going to fire at full power, or even 1/2. So the extra 2/3 a stop really won't make much difference.
I shot an event saturday night with on-camera flash, but I use a very different set up. I was in manual everything, camera and flash. I set up my flash at 1/4 power and zoomed it in to 105mm. i pop up the white card for a little more fill when shooting landscape mode. I was using my 24-105 at f/4.5 (i think) and around 2000 ISO. I stand back and zoom in so I'm rarely under 70mm.
I bounced the flash off 18 foot ceilings all night and it was a great balance of ambient and flash.
I have a 580 EX and a smaller Nikon SB-28. I brought the Canon flash in out of habit and never had time to run back out for the Nikon, which would have done the job just fine.
Anyway, I understand not liking to have a heavy flash hanging off the camera. For me, it really makes a difference when holding on to a camera for a few hours and the smaller the better. I'd rather carry a heavy lens than have a flash throwing off the balance of the camera.
if you aren't bouncing the flash, the extra power is definitely not coming into play. Well unless you are shooting at super long distances.
Just another opinion to throw into the hat.
PSA: The above post may contain sarcasm, reply at your own risk | Not in gear database: Auto Sears 50mm 2.0 / 3x CL-360, Nikon SB-28, SunPak auto 322 D, Minolta 20