The wisps at the top of the cloud don't indicate that it's building rapidly.
I agree. But they do mean that it is a
cumulonimbus from an observer's point of view. The point, which you make very eloquently, is that the observational distinction between CU and CB does not offer a definitive guide to the aviation hazards in the cloud.
I got a sharp reminder of what you write about TCU on the way into Lille a week or two ago. The flight had been almost entirely smooth at FL100 above scattered little CU, but 50 miles out there was a cauliflower CU with its tops at about FL110 or FL120. That's really not very high, is it? I figured I was going down anyway, so hey, what can a little CU like that do? Five minutes later, my wife simply said "don't do that again please".
Looking on the radar after, it seems I picked the nastiest little shower over northern France at the time, with turbulence that was on the sporting side of moderate. Welcome to Spring 2009!