Some useful comments from facebook thread about this section:
Fabian Simon: "I can confirm the WW grade class III. We paddled it one year ago after a long day of kayaking as an evening run and were surprised that it was that easy.
But if you paddle it with more water, the river will not be the same because in the narrow gorge, some cubics can change the height of the water really fast (also because of some hydroelectric plants at the Grimselpass more upstream).
The best would be scouting the whole section from the tourist path in advance, then you can be sure that there is no tree or similar in there ;)
Regards Fabian"
An Di: "many years ago we paddled the Aareschlucht with about 50-60cbm. I´wouldn´t do this one more time..., a friend lost his boat in a undercut, 3months later it was found in the Brienzer See. On high level the narrow gorge is very dangerous, at low level it´s really WWIII. Don´t forget that just at the beginning of the Gorge is a powerstation... the flow in Innertkirchen is not the flow in the Gorge!"