Two things:
1. I can tell Turn10 didn't spend as much time on some cars as it did others because the interior sounds on a LOT of cars are the same as the exterior sounds. Granted, in some cars, that might be accurate, but when ALL the BMWs I've driven only have very slightly muffled exterior noise sounds, it gets into the stupid. Even GT at least muffled the engine noises more than an decibel or two. It makes no sense.
2. The Prius. So, in GT, if you fully tune the Prius, they take out the batteries and you have a pretty decent sounding race car. In Forza....they keep the batteries in and you have a car that sounds like a snowblower. Yippie. I would LOVE it if FM5 allowed you to remove the batteries from the Hybrids. It reduces the weight, and gets rid of crappy engine noises.
EDIT: On the Porsche thing: If I remember correctly, Turn10, at the beginning basically said that EA was stupid and they REALLY didn't want to pay them money for the Porsche license because they, themselves don't believe in exclusive licenses because it's bit unfair, hence why they gave up their exclusive Ferrari license. They wanted a fair, even playing field. Dan even said that he likes seeing his favorite cars in every game possible. But, of course, the Forza community whined and cried over not having Porsche, and MS gave in and forked out the money to EA.
PD has never really been one to push for a license. I believe Kaz once said that they have so many manufacturers that with every game, they simply ask a make once to be in the game, and if they say, no, they don't have time to argue. Which is also probably true with Turn10. Although, they said they were bugging Porsche constantly about their license.
That said, Porsche didn't renew their exclusive license with EA. (This might be part of EA's legal issues with having exclusive licenses, actually. They got sued by 2KGames for hogging the Football licenses, and I think part of the settlement might have been that EA can no longer have exclusive licenses. Of course, the courts let the contracts run out, but...EA doesn't play well with others.)