You are surely not suggesting he just turned up in 2006 and then developed the existing car for the next 7 years??Didn't Adrian Newey design mclarens 4 years ago?
Edit: appears he started designing red bulls properly for the 07 season having left mclaren in 06 and was given an already designed car to work on when he got there.
Typically F1 cars are developed by parallel development teams within a team, so that while one team is working on this year's car, the other team is working on the year after's car, but with somebody at the top overseeing overall development.
When Newey arrived he would only have been able to influence the 2007 car at best.
Then Red Bulls of 2007 and 2008 were reasonably fast but very unreliable. It is a major part of the job to not only designs a quick car, but one that will finish, and that means a huge overhaul of the engineering side of things.
Then of course there were the regulation changes to contend with. The huge change of 2009 say the first of the current concept Red Bulls, and that car would probably have won the championship, but for the famous "double diffuser" that Ross Brawn and Williams managed to design and get away with. I am sure that if McLaren had turned up with that, as a major team it would have been banned, but as there was a feeling of supporting the underdogs,, Brawn and Williams were allowed to run it.
From then on Red Bull played catch up, and eventually were quicker by the end of 2009. 2010 onwards are now the stuff of records.