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I think turbocharged vehicles vary much much more and you would have to study the stock tune to get a feel for this.
Good rule of thumb is the highest load where it's still stoichiometric is the most efficient load. Some turbo engines can stay stoichiometric under boost (newer ones), while others run rich quickly. You can get much more mpg by leaning out the fueling under boost.
On a diesel it's hard to tell because combustion efficiency is much more variable.
With VVT it is possible to trade torque for efficiency. You can delay the intake valve to get "simulate Atkinson cycle", which decreases VE and allows you to open the throttle fully while not inducting as much air. I don't think this is a common setting since I imagine response would typically be prioritized, but I have no idea.
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