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@hamrack 3N80 can be serviced too, the guts are available separately as I found out when mine failed. Think I payed €30ish for a kit and when you take it apart you can access the bearings. Quite hard to do if you want do get at the bearing specifically though, there's a wire that's prone to snapping during the process and then you need new innards anyway... Here's a link but it's €41 now apparently: http://bicikli.de/shop/SHIMANO-spare-part-Y-2Z498010-DH-3N80-INTERNAL-ASSEMBLY-AXLE-LENGTH-108MM-Shimano-Code-Y2Z498010
So reading off the data provided for the dynamo without an electrical load, the SON28 uses ~1.5 W at 30 kph and the 3N80 uses just under 3 W at 30 kph. Likewise, it's ~6.5 and ~7.5 W under their test load at 30 kph. That's where I got my 1.5 W difference.
Schmidt's own testing has a non-dynohub as a control and they reckon 0.5 W at 30kph (with the SON28 at 1.5 W and the 3N80 at 2 W).
Either way, my point was that the SON is glorious but you don't save that much in power terms (although you gain in reliability as the Shimano isn't really designed to be serviced).