Re PPD increase vs. power increase: It very much depends on the point in the frequency/power curve at which you drive the card. I guess it should be possible to get more PPD/W out of 1080Ti at optimum operating point (compared to 1080 at its own optimum operating point) in F@H with its bonuses. But maybe not in BOINC projects with linear points/work.
Finding the optimum op point is difficult though: AFAICS it involves playing with power target, clock offset, and fan speed. (OK, the last one is easy: Most is best for GPU perf/Watt.) And then measuring PPD takes a lot of time in a project like F@H where you get wildly differing work units.