the point is:you need to push the car to the limit to get any information of value.
I expect all teams are doing this otherwise they start all over again when the real thing starts.So in effect you need to know if people deliberately lift at certain points of the track to mask their performance ,you need to look at fuelloads and tyre choice.
Additionally the track conditions play into this .
A team could even go as far and put ballast into the car to mask ultimate performance..mind you with the lowish temperatures you might be looking for setups not useful for hotter conditons .The additional load would also help to establish more relistic cooling demands.
Whitmarsh stating there was 2 seconds in lap times left for the big teams is just stating,they are not running the car yet on qualy fuelload in fact they go for a medium load ,as they have effectively considerable variable there you will never know for the last tenths,maybe not even second.
a clear indication is some teams -sauber,Williams,ForceIndia,TR,Marussia look better than you would expect ,but thats simply the tyre choice and fuelload,maybe even ballast