I'm a firm believer in "fast out of the box", which is why I immediately believed in people like Kimi and Alonso the first year I saw them racing (same was true for a young Prost, Schumacher and Bellof). I don't believe in gradual success and that given enough practice Rossi will make up the big time gap. Of course I could be completely wrong.
How many other open wheel formulas have KR,FA etc run and won championships in ? These guys have been driving these cars for years that's their career. Rossi has tested only 3 or 4 times in his life. I'd say he IS pretty fast out of the box IMHO. It's an apples to oranges comparison at this point.
But they were not "out of the box" as you put it, they all had years and years in karts, F2000 then F3/000. Even 10 seconds off the pace is pretty damn good. Just look how Michael Shumi did when he tested the MotoGP bike a few weeks ago. He was much slower on the bike, then Rossi was in a car.
Well, that's the problem. There is no time for Rossi to learn to drive on 4 wheels. That train has left the station when he was a toddler. Just like MS will never be competitively fast on 2 wheels. Rossi in a F1 is just a dream.