This old article had the 24s being really very aero give their depth...http://www.rouesartisanales.com/article-15505311.html
I've had the C35s (7900), but never the 24s. 35s, are great wheels, durable and stiff, but I don't think they are much to right home about on the aero front. They are still a preferred wheel on the CX circuit. I think they are 21mm wide, not sure if they've changed the rim for the 9000 version, but I think not. Fwiw, I can run an 11 speed 6800 cassette on my 7900 wheels by removing the 11 and adding a 1mm spacer behind the cassette.