I'm by no means giving PD a free pass. I've almost given up on the series, to be honest. However, I will acknowledge that the Cell processor in the PS3 was notoriously difficult to optimize. I'm sure PD went in seeing the specs of the system and optimized towards that. Then they found how difficult it was to utilize the system to its fullest.
A lot of the major problems with GT6 (and GT5 for that matter) are not hardware based though.
Sure, it has unsteady frame rates and flickery shadows, and those sorts of things
can be solved with more power. But the weird design choices can't necessarily be.
Rubberbanding AI isn't a hardware constraint, it's a design choice.
Likewise a physics system where camber and tyre pressures are non-functional.
Likewise a career mode consisting of dozens of short races where you have a rolling start from the back.
Likewise an online system that is bare bones at best, not including some of the better features found in GT5/GT5P.
Likewise the decision to include hundreds of low quality last gen assets.
Likewise the decision to remove features found in GT5, replacing them with nothing.
There's lots of things where I just think "and this makes the game better how?" Nothing to do with the power of the hardware, just odd decisions by Polyphony to limit the choices of the player, mostly for reasons that are totally non-obvious.
I maintain that GT6 has all the tools that it needs to be the best racing game ever, if only those tools were presented to the player correctly. For just about every problem I've found with GT6, I have been able to think of a solution that doesn't require more hardware power. Maybe there's some reason why those solutions wouldn't work, but still.