I really don't see how the very same chips are effecting different consoles differently. Even if you shrink the die, the chips logic is still identical, the software could not run otherwise.
What you are missing here is streaming. I have seen textures disappear at bathurst then reappear (its rare though). The PS3 is not fitting everything into RAM, you are getting textures when you need them off the HDD it seems. I think what you are seeing is overhead related to texture management in connection with HDD speed. And your tests correlate with this. I don't have most of the issues you speak of, yet my console is a 40GB with an upgraded HDD to a 7200RPM 500GB. According to your tests my SKU should be bad, but its not.
I have the console you listed as the worst. Dealer animations are silky smooth, I never realised people had issues with them. The only difference between the console I have and the one you tested is my HDD upgrade.
Also you must realise that when you are buying a car the game is not only loading that car into RAM, it is also installing it into the HDD for next time.
The thinking is that thermal issues are throttling the chips, because yes, they can all do the same number of ops per cycle and have the same cycles per second - except that the Cell scales the clock speed according to how much work it has to do. Not much to do? Scale back to save power (which equals heat in this case).
The theory is that the temperature monitoring built into the silicon can do the same scaling: getting too hot? Scale it back to reduce power; reduce heat input, reduce temperature, keep things solid-state. Scaling the clocks like that at full load will introduce a slowdown effect, as it'll take longer to do the same ops now.
The key point is that whilst the chips are logically equivalent, thermally they are far from equivalent, even within the same "model" (since some chips will need a higher voltage to hit 3.2 GHz than others, due to variations in the manufacturing process - and power is proportional to the
square of voltage). Add to that the various coolers that Sony has used over the different models, which have caused problems for some owners, and it's clear that there is plenty of scope for the thermal balance of any model to vary, and to vary between models, too.
That implies a different thermal sensitivity on a
per-console basis in terms of computational load, hardware (inc. cooler) and environmental factors, not least of which:
dust. So we're looking at which models are
more susceptible to finding themselves operating out-of-spec., thermally speaking.
We may find that the frequency of "thermal imbalance" can be pinned on different things for different models; e.g. perhaps the smaller dies suffer from rare 1 mm (say) bubbles in the thermal paste applied at the factory, which constitute a much larger relative area than with the larger dies (using the same process), and thus cooling is more adversely affected when those bubbles occur - relatively speaking. Such a thing shouldn't be possible (at least, not on that scale), but you get the idea. Maybe the fattys are more susceptible to dust because of the higher thermal loads (again, just an example to illustrate) etc.
The harddrive does indeed play a role, in a "full system" sense, but why would it affect the dealership rendering? Mine developed a mild stutter in the dealerships that wasn't there when I first got the game, and now it's gone again.
It's obviously a very complicated set of interrelated effects, which is why you can't say that such and such model will (always) do X. Rather, what we can potentially achieve is to say: if you're getting X, try Y. If it's possible to characterise in that way, even.