Brawn had to shorten the car to fit the Mercedes engines in. Also saying that the double diffusers is the only reason Brawn was dominant early is like saying Red Bull was dominant because of their flexy front wings. Only one trick does not make a magic show and it takes more than one highly developed area to make a car so dominant, when all the pieces come together and work in unison then that is when a car is at the top of it's game.
Then explain how such a competitive package translated into a very shitty Mercedes 2010? Honestly that double diffuser gave them half a second a lap of advantage to everyone else. It was a lucky strike and nothing else, it was Honda for crying out loud, they've been trying since the mid 2000s to be competitive and they failed, hence they pulled out. It's not lie they had slowly built up their performance and it peaked in 2009 as Brawn - they sucked.