You said, "all else being equal." I, however, did not. What I specified was, "higher compression, smaller displacement, same performance." The Lamborghini has, what, 2.5 times the displacement of the Civic? And the Lamborghini has, what, about 3 times the horsepower? (I'm ballparking here because I'm too lazy to look up the real numbers, but I'm reasonably confident that I'm fairly close to the true numbers.) So, the two are not comparable because the higher-compression engine has a larger displacement and much higher performance. A more correct comparison would be between two cars with similar horsepower.

(EDIT:I looked up the numbers, and I was damn close with my ballpark estimates. The Civic engine is 1.6L and 115hp, the Lambo was 3.9L and 385hp. Unless I looked up the wrong Civic... but then I'd still be reasonably close. )

I don't know whether for the same displacement, a higher compression ratio will result in lower gas mileage. It is possible that the higher compression would require a richer fuel/air mixture, thus leading to increased consumption and lower mileage. It's also a function of how fast the driver accelerates, of course, so I'm not sure there's an easy answer to that one.