Yeah I'm not so sure about that $300,000 and 500 credit score comparison to $30,000 and 780...I think it's bullshit. Jesse's right, those are pretty outlandish scenarios, given that the person with a credit score of 780 living on $30k/yr probably knows how to budget and manage debt pretty well and wouldnt be in the market for a car so far out of sync with their lifestyle. The guy earning $300k is a less attractive borrower because that small percentage that he fits into has a marked propensity to spend beyond their means and attempt to finance before they have earned, and his low credit score is indicative of this. Not making payments on time, etc leading to poor credit score is an early warning sign.
This is not going to include all possible cases, just some generalization, proving that credit score DOES matter. Why do you think that engineers, while not the top wage-earners, are often more likely to get a large loan than managers? Could it be that they have better job security, and are therefore more likely to consistently pay back the money? There are people making 6 figures with Porsche in the driveway without $5 to put in the gas tank...and they probably miss their damn payments too.
Your thread got jacked, but I think you got an answer up there somewhere!