Mainly the part where you say they're reevaluating and rebuilding their method from the ground up. Earlier, I've seen you compare their efforts to Sonory's... Where do you get that notion?
He's doin' just fine
We have a great means of comparison, because several cars use the exact same samples in GT6 as they did in GT2. So you can track the progress of how those samples are used over the successive versions of the game.
Very simply: although their samples have been carried over, the audio engine has been re-built for PS2 and then again for PS3. That's the opposite of what usually happens: the samples are re-designed and the engine carried over.
There are a number of impressive physical considerations implemented in the source mixing and localisation in the PS3 outing that imply a fundamental re-design of the sampler controls and buffer management - namely: the sound delay and "filtering". I don't want to go into too much detail, because it gets very technical very quickly, and although I like to believe that it's easily understood by anyone given the time, the chances are no one will care!
Other developers have tried these things, but gave up when the immediate changes didn't result in something that sounded good: you have to push past that with more challenging of assumptions and re-evaluation of base knowledge. Many of the sound quality issues can be avoided by starting from scratch, rather than trying to "retrofit" an existing system, since the "mistakes" are more obvious and the correct behaviour more intrinsic. That takes time, effort and money, of course, which is generally "better" spent elsewhere in the minds of those focused on the short-term - the bane of any ambitious developer.
PD demonstrate such re-evaluation in other areas of their work (mostly graphics, naturally), and Kaz is often heard saying they re-built things in terms of the base assumptions.
Note that I was not comparing their work to Sonory's as such, I just used that as an example of a successful, purely-synthetic engine sound "technology".