Upload & Sell: Off
my SB 910 is not strong enough to kill daylight further then 1-2 meters away, let alone with some kind of diffuser
Sounds like you're using high-speed sync.
I use Canon's version of the 910, the 580. Granted, I think Canon's flash is rated just a tad more powerful than the 910 but I am able to fill shadows on a sunny day with a subject distance of three to about six meters. [So ten, even twenty American feet.] I avoid high-speed sync, use no diffuser on the flash, and do use a neutral density filter.
A quick search brought up the quote below off the flickr photo-sharing site, which was in response to someone who was considering purchasing a flash with high-speed sync capability:
"If you want to overpower the sun, you will have to stay at sync speed, because you lose too much power with HSS - believe me."