+1
I also think improved efficiency claims are mostly marketing fantasy.
I'm a convert to B-vee belts instead of serp. B belts have a wide enough section on the inside to ride flat on a CS type flywheel, and the cheaper B-Vee pulleys don't require as much belt tension, so generator bearing life will be better. My serp setup (without idler/tensioner) requires very high tension to avoid chirping. I used a B vee on my neighbor's DES 8/1 because I wasn't confident about our final engine rpm...now I'm sold on them.
I think serpentine belts are great for what they were designed for, multiple driven accessories.
They save a lot of space and some cost in that application.
Slotted bases sure are simple to build, and belt adjustment via loosened bolts and rubber mallet works for me. I haven't adjusted mine for years.