I normally ride on 32 mm Hutchinson Acrobats which are decent overall but they have none of the features that make for low rolling resistance in a tire. My LBS recently let me borrow an old pair of Michelin Pro2 Race tires, 25 mm. I took them on a 40+ mile test ride. Here is where the real world interferes with science. On the test day the wind was awful and the limestone portion of the bike trail was soft. I think the tires were more efficient, but it was impossible to be certain under the conditions. Even so I am looking for a good tire for a century attempt and I went ahead and had him order me a pair of Pro3 Race tires in 25 mm, the Pro3 is the current model of the same tire. I haven't mounted them yet but I am running the 28 mm Vittoria Zaffiro's that came with the bike as a training tire. Both the Zaffiro's and the Pro3's measure 28 mm wide on my 18 mm wide (internal) rim. That may seem odd but the Zaffiro's are probably designed for a rim that wide while the Pro3's are probably designed for a 14 mm or narrower rim.
I'm not looking for speed, just efficiency. I want to finish the century and anything that will help me do that is on my short list right now. The faster you ride the less speed advantage you get from an efficient tire because the aerodynamic power requirements are going up with the third power of speed while the rolling resistance power is only going up linearly. So, in a way, efficient tires are helping slowpokes like me more than professional racers, it is just that a racer cannot give up any advantage, however small, or he/she risks losing. I'd buy a 28 - 38 mm Pro3 Race in a heartbeat but the high quality tires are made only for racers (I guess they think the rest of us don't care about efficiency) and at race speeds aerodynamics matter a lot so 28 mm tires are quite rare, 25's are a little hard to find, and most are 23 mm and down. I'm only going with 25's because I can't find anything as good any wider but I am quite happy to find that they measure out at 28 on my rims!
Ken