Originally posted by kunik If you don't have anything to bounce off of (i.e at an outdoor wedding) then the "diffuser" is just sending the light out into the atmosphere. Any light that hits your subject is originating from the same size light source. If you think the lighting is improved when you put on a diffuser outdoors you should just dial your flash down because that is all that is happening - less light is hitting your subject. That way your batteries will last longer too.
Kunik is right about the theory behind the Sto-Fen Omnibounce. Outdoors, an Omni-Bounce won't do much (although I don't think it does nothing). I'm not sure why I mentioned the Omni-Bounce in the first place; I think I forgot that the reception is outdoors, too.
The entire purpose of my original post was to try to my recommendations simple. If shooting outdoors, using the 540 FGZ straight forward, P-TTL, no diffuser or modifier, is probably the safest bet - the approach least prone to complete failure. Shooting parties outdoors at night with a single, camera-mounted flash is just plain difficult any way you look at it.
Will