VW screwed up their 1L when they failed to put a V8 in it!
I know you're being sarcastic, but I actually would agree with that statement. Given that the 1L is an extreme, a V8 would drop FE by 50% or more due to such a low load operating point for normal use, but even still, the performance and still high economy combined would be impressive. It wouldn't be a 250 mpg miser, but a 100+ mpg miser with a V8 would still be nice.
But if you have less extreme cars, say, a V6 Mustang(22 mpg combined, 210 horsepower) versus the V8 Mustang(20 mpg combined, 300 horsepower), or the L4 Camry versus(27 mpg, 158 horsepower) the V6 Camry(25 mpg, 268 horsepower), the differences in FE between the les powerful and more powerful versions are around 10%, while the more powerful versions tend to have horsepower increases over 50%. If you look at the same models of car, truck, or SUV and compare engine options, the difference in FE between the most powerful and least powerful models isn't that great, while the difference in peak horsepower available is huge.