Vacuum advance

Great explanation A/J.
I'll disagree with this part, and only this part.
It's important enough to discuss.

AJ/FormS said

At idle, it is desirable to reduce the power by delaying the pressure peak, to smooth initial transmission engagement, and to smooth drive-away on the fluid-coupling at Part Throttle. Yes the engine will be snappier with more pressure, (created by earlier timing)

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We can always use more power at idle. For sake of this discussion, I think we are all on the same page that we're only concerend with the maximizing power and fuel efficiency. So we will accept that maximum power requires rich mixtures, and this results in lots of CO and HC.

It takes a relatively rich mixture to get an engine to idle at a low rpm and not die when engaged with the transmission (through torque converter or a clutch). This is because we are asking the engine to produce close to maximum power at that rpm. Maybe it takes 10 hp for the engine to run at 650 rpm. But it takes maybe 40 hp when the clutch engages and put the car into motion. Run an engine lean and it can only make 20 or 25 hp at 650 rpm. We see this lack of power when an engine is placed into drive (automatic trans) and the rpm and vacuum drops.
With an automatic in drive the engine its not even moving the car, and yet we sometimes see engines slowing down and stalling.
This didn't happen with 'low performance' factory engines. Why? One reason is the high performance engines often have less torque and power at the same idle speeds. Another reason is the hot rodder has set it up to run lean at idle. This only works well if the goal is to run in neutral.