Some good news: I bought a new set of the Marathon Efficiencies for my wife, and after a preliminary test, they aren't slower than the Supremes, and may be a smidge faster (which corroborates the data from bicyclerollingresistance.com)
Previously she had:
Front: Schwalbe Marathon Supreme 32-622 (360g)
Rear: Schwalbe Marathon Supreme 38-622 (440g)
Now:
Front: Schwalbe Marathon Efficiency 40-622 (520g)
Rear: Schwalbe Marathon Efficiency 40-622 (544g)
The "test" was a 43-mile loop under similar conditions and effort levels, mostly on smooth asphalt, some slightly-rough asphalt, and a 15-mile stretch of well-groomed gravel rail trail (the Snoqualmie Valley Trail outside Seattle, WA). It's mostly-flat, but has several 50-100 ft. up-and-downs, and a 200 ft. 4% climb on gravel.
I always ride behind her, and generally when we coast down hills (and I move clear of her draft), the extra weight of me + my bike means that I need to use my brakes to keep from passing her. The first thing I noticed with her new tires was that I could brake less (or not at all) without passing her, suggesting a faster rolling. At a minimum, the Efficiencies didn't roll slower than the Supremes.
This is in startk contrast to our disappointing experience with the Almotions a couple years back (partly inspired by bicyclerollingresistance.com's super-low 17.1 watt measurement vs. 19.1 for the Supremes), where, with the Almotions on her bike, I definitely needed to brake more on the downhills vs. the Supremes. At the time, I attributed it to switching from the narrower 38-622 Supremes to the wider/heavier 40-622 Almotions, but now that the same 38->40 transition with the Efficiencies didn't result in the same downgrade, I'm now pretty sure that we ended up with the Addix-compound Almotions (tested at 19.4 watts) rather than the OneStar version that BRR initially tested.
Our average speed for the 43 miles ended up being 12.75mph with the Efficiencies vs. 12.62mph with the Supremes. It's just one test, and it's not a huge difference, but again, the heavier (by 264g) Efficiencies weren't obviously slower than the Supremes.
And the main reason for trying out the Efficiencies was the hope that their shoulder blocks and extra width/volume would give my learned-to-ride-a-bike-at-age-35 wife more confidence on gravel than the entirely-smooth Supremes. The rail-trail was a bit too smooth to make a great test, but she took an up-and-down section of loose stuff crossing a drainage about twice as fast as her previous two traversals, and made it through an unexpected section of new loose stuff upright that she said she might have lost it on with her old tires. And when I was testing them out myself on some hiking trails, I powered up a 15 foot ~20%-grade rough-gravel hill without any slipping, something I'm pretty sure I couldn't have done on the Supremes. They also felt qualitatively more comfortable over bumps to me (not surprising changing a 32-622 to a 40-622), though I don't have a lot of experience on her bike to be sure.
So getting those extra benefits without any performance loss would be a genuine win. Of course it was just one test on one day, so far from a final conclusion, and we know nothing yet about puncture resistance and durability, but it's at least an encouraging start.