Time for more speculation! 'cos it's easier to sit here and speculate than go and fix my leaky distributer...

So, in the late 80s Toyota were scared about the introduction of unleaded and they 'backed off' the ECUs. Thinking about this, what would they have actually done?
If you are worried about combustion chamber temps you aren't going to reduce fueling, if anything you'd make it richer. (Not that I fully believe the old story that lean mixtures run hot and rich ones run cold.) So, I reckon you'd retard the ignition. Especially at high revs where it's more likely to suffer detonation (pinking). And that would cause it to be less rev happy and feel flat.
I believe one of the early techniques for a rev limiter was just to keep retarding the ignition until the engine wasn't producing any power and the driver had to change up. I don't suppose you can do that with catalytic converters so they just cut the fuel off.

Two things I believe Jeremy Ross said (although I'd struggle to find the quotes)...
First he suspected that the 17070 was intended for a different territory. So perhaps Toyota, suddenly afraid of fuel quality in the UK, went and rummaged in the parts bin and found a suitable ECU that would do the job until they did the proper facelift later. Hence we get the 'crossover' cars.
Second, I'm sure he said that the early 17030 ECU had a fully 3d mapped ignition all the way to the red line. But the later two ECUs were only fully mapped to 4500 revs (or something like that). After which what are you doing? My guess would be that you ignore the revs and pay attention to the MAP sensor. But I'd ned to think about that...

I think it's a shame Jeremy disappeared too. I think he'd be a fascinating chap to get in the pub for an evening.

James