Just looking at Qb rating is not a great way to measure QB's for several reasons, most notably the fact that TD passes are represented in the formula. If Losman passes it all the way down the field (hypothetically) and McGahee runs it in, why should his QB rating be any less? Qb rating is especially useless over just a few games data. All it tells is if a guy has a 90 rating he's played pretty well, and if a guy has a 60 rating he sucks.
Yds per attempt seems to be the about the most accurate measure of a QB (with a peripheral eye on INT percentage.)
The great ones do well in this statistic, Kelly was over 8 in his great seasons, and this is why I'm getting more and more sold on Losman.
The fact that he's throwing it down field and accurately shows poise, confidence and improvement. You can see these things - they are obvious - if you watch him. If he had completed 68% of his passes at 5.8 yds/att with a 95 Qb rating, I'd be a lot less encouraged.
Losman had a better preseason than most of those QB's you mentioned, including Brady.