2% apparently: "
There's no noticeable noise from the gearbox beyond a quiet purr, and no noticeable extra drag or effort required to pedal. Efneo has tested the drag at high loads and estimates it at around 2% – a perfectly acceptable loss in my book and on par with any other internal gear mechanism.
As for manufacturing: "Manufacturing of the Efneo bottom bracket and the parts for the gearbox is done by Tange Seiki – a Japanese company with nearly a hundred years of precision cycling component manufacture behind it. Final assembly is done in Poland."
https://road.cc/content/review/22779...-front-gearbox
As for using a chainring < 30T: I used a 24T on a double crankset with no issue, so a 28T should be OK.