Newey went as far as stating Mclaren got a better handle on matching their car design to the requirements of the pirelli tyre and attributed (in RE a few months ago)the advantage of RB7 to abetter general design going as far as suggesting they would have had a bigger performance advantage if Bridgestone tyres were still in use.
Mind you Mclaren were the team testing the old car longer than anyone else which was giving them more design time and later feadlines but they paid a price when they had to accept that a key performance system was flawed and proved unreliable.
I certainly believe you can move deadlines IF you are sticking to your concept and just optimise and adapt in those areas that are required by new rules ,or a new supplier...and if you know your base is competitive..In Mercedes case all this is just not possible,they tried a lot of different directions and finished the season
with an uncompetitive car .As the season unfolded W03 came out as the second best mercedespowered product and considering the teams recources it is more than likely that w03 had even less ultimate potential than Force Indias design as the gap came down to zero ..and who would realistically value Sutil/diResta higher than rosberg/Schumacher not to speak of the sheer workforce and recources available to a works outfit.
So they need to make another BIG step and that creates the need to be ready early to have the opportunity to iron out flaws .