Why do you not believe it is possible? I believe that you are looking at Beyond the apex too literally. Those 6 months per car can be done in under a week if you look at it properly.
I know that in my company when we refer to man hours in that long of a time frame we calculate it based on five day work weeks. So 6 months would be roughly (averaging 21 business days a month) 126 days or 3,024 hours. Now lets say you have a different guy working on each part of a car and it takes them 3 days to get their single part completed. That comes out to 42 guys using 6 months worth of man hours in just 3 days. If we want to assume calendar months, that is (averaging 30 days a month) 180 days or 4,320 hours. Still, that would only require 60 men to do six months worth of work in 3 days.
I know I am simplifying it for my example, but Polyphony has a moderately sized team and that six months does not mean that the entire team works on a single car for six calendar months. If it did we wouldn't even have Prologue yet.
I am a manager over 15 people, so in a five day work week my department does 2.5 months worth of hours.
So, to me that figure is not that unbelievable, especially as people do a specific job over time they get better at it, make less errors, and work at a faster pace. Efficiency improves over time, and without knowing where in the development cycle that six month figure was referring to, or if it was even just a rough exaggerated estimate to sound impressive I don't know if that six month figure was peak efficiency or early on. If it was early on I would not be surprised to find that they have shaved anywhere from 10%-25% off of that time.
Did you seriously compare an HD upgrade of an older fighting game to GT5? The differences in what is required here is staggering.