I would be VERY surprised if Mclaren aren't there/thereabouts come Australia, none of the teams ever show their real promise in testing.
The Brawn GP team are obviously reasonably quick, but remember they're dying for sponsorship so will be going all out to prove their pace with light fuel loads, high downforce - anything that can help the car be quick over a single lap or so.
I'm not saying their pace isn't genuine, but they won't be faster then everyone else at the first race, no way
I hope they do well though, i know Lloyd Story works there and he said it was pretty bad over the last few months so it's those guys i feel for, as for Jenson I think he's an ar$e - all i've ever heard him do is moan about the car and how he would do so much better in another one. He chose money to go to Honda / BAR so only has himself to blame.