Well that's one opinion. The noise rating is worked out based on a naked bike ( from memory ) and I'd have to disagree completely with your summary.
I've always had Shoei's and AGV's and my 2 current full face helmets are both Shoei but the C2 is up there with them in terms of quietness. I ride the GS, a BoxerCup ( 1100S ) a 1300 Pan and a SV650 and the C2 is nice and quiet on all of them. To say it's only quiet on massive fairing bikes is frankly utter nonsense, helmets are a very personal thing and your head shape plays a fair part in it.
I've heard more than one person complain about misting though I seem to have been lucky and escaped any problems with that. It's ugly certainly, but noisey, not for me and I'm really quite fussy about a quiet lid.
The GW or LT thing is just to point out that the wind noise is from turbulence, which you don't get from GW or LTs. Also, if your head is sufficiently below a windscreen, even on a GS, you'll not have this problem.
In fact, several of my colleague sites have reviewed the C2, notably webBikeWorld in the UK, and came to exactly the same conclusion.
As I've said, I've got some 7 helmets, most given by manufacturers for testing, and from all the full face, or flip helmets I've got, the C2 is by far the most noisy.
The noise is not from the shape of my head, but from the rear entry/exit vent, not the top. I tried taping shut the top vent, and that did not change anything. It's the wind going around the helmet to the rear.