My belief is that there is no substitute for track time. The data collected during testing, practise, qualifying and races cannot be accurately modelled.
Virgin have shown this categorically by overall being beaten by a car that was only really started in September 09 (lotus) and a car that wasn't run till the first day of quali (hrt)
Red Bull and Brawn in previous seasons have shown some deviation from this trend but both had either put a lot more effort into the car before hand or had a good base to work on anyway.
There's a very good reason teams that can pound in the laps at every test look better in the races and overall do better than those that get bogged down with reliability or miss track time