Quote:

2. You run less timing: while under light load (vacuum or low boost) the timing is the same-ish as an N/A engine, when under heavy load (high boost) timing is retarded 10-30 degrees easily, obviously having large impact on efficiency.




Actually, this is not as cut-dry as that. I hope I can explain it accurately...

Ignition advance is required because the mixture does not explode instantaneousy @ spark, but rather as a controlled burn that takes time. As we don't want it burning much after TDC, we must start earlier so that we have complete combustion for the start of the power stroke. More timing is needed as RPM increases becuase there is less time available for the burn event (which still requires about the same amount of time, in comparison).

The flame front travels faster from droplet-to-droplet in a denser or more homogeneous mixture (I believe this is an accurate statement).

So, the more efficient a cylinder head (think turbulence, swirl, quench) or the more boost you have, the less timing is needed, as the mixture is more homogeneous, and gives better flame travel. that does not necessarily mean the burn is less effective.

Excessive timing lead causes pressure to build on the piston while it is travelling upward on the compression stroke - reducing efficiency.

To summarize:
I don't think we can really compare timing #s between NA and boosted engines as relating to efficiency.