Kaz said that 60% of their effort went into the cars for GT5. That's not 35 people. Either Famine has an old figure, or PD has a lot of people not contributing directly to the game.
Indeed, the long ending credits have 48 car modelers and 5 modeling "directors".
There are 27 "landscape designers".
A quick scan of the names didn't immediately suggest any overlap there, either.
So really, taking a figure of 50 modelers, it's more like a car every 3.7 days, or just over 40% higher throughput. That's just over 100 cars per year.
Something else has soaked up some of that productivity, because we don't have 300 extra Premium cars since GT5 launched.
EDIT: If I were to pull a figure out of my arse, it would be that a Premium without an interior takes half as long as as one with. Would that mean that 200 cars have been externally Premiumificated?