The noise attenuation in a DC Helmet will be as good as the DC headset you put in it, plus a dB or so. Whilst the blurb says they will fit standard visors, what they mean are the peaked-cap variety, rather than a normal helmet visor.
The nape fitting on the DC helmet seems designed to restrict neck movement backwards in normal use, and completely at odds with the cut-back nape design of the Gentex HGU33/55/68 shell - I originally bought DC helmet shells, but wouldn't wear one now given the choice.