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What you are forgetting is that there is more than one way to downsize an engine other than physically.
A turbocharger mated to a late-closing intake cam will see your fuel efficiency increase, guaranteed. The only sticking point is that the turbocharger has to be more efficient at compressing the intake air than the piston during the early part of its stroke.
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If you could vary that intake cam on the fly, I could see your point, but why would you want 2.5L worth of metal moving around in a motor that breathes like a 1.8L? So the turbo brings you back up to stock horsepower? Also remember that turbo energy isn't entirely free. That backpressure costs you.
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Also, I don't understand why a turbocharger doesn't decrease pumping losses, at least on the intake stroke. If the intake manifold is above ambient, doesn't that mean the absence of pumping losses?
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The manifold AT ambient decreases pumping losses and any engine can do this perfectly well without a turbo. It's called flooring the accelerator. The problem, like I've stated many times in this thread, is that I have no way of efficiently using that power. The way a turbo can help, is if I downsize my 2.2 to something like a 1.2T. That way, I can run that 1.2L at atmospheric manifold pressure while cruising, well within the BSFC sweet spot. The only reason that turbo is necessary, is to make the little 1.2L viable for when I need to pass or merge into traffic. During FE cruising, it lies dormant. If I run my 2.2L at atmospheric manifold pressures (with turbo or without), I'll be over 100mph before long.