I have a pair of the H8's. I ordered them, pre production, so they are among the first ones available.
They are definitely C&O versions, and quite accurately detailed. Rivarossi made some changes to their mechanisms, as a "reaction" to Bachmann's introduction of the Spectrum 2-8-0. The Big Boys, and Challengers were upgraded, and were much better runners than before. The same mechanical upgrades were used in the Allegheny's. They run better than the earlier Rivarossi products, but not as well as the LifeLike P2K Heritage, or the Bachmann Spectrum. The Rivarossi drive isn't as smooth in slower speeds.
I have a Soundtraxx Tsunami, heavy steam decoder, that I will be installing in one of my H8's.
I also have one of the improved Challengers (D&H). I was able to improve its performance by adding wheel wipers to the tender trucks, and got better electrical pickup.
All things considered, The Rivarossi Allegheny's are worth getting. I don't know if Hornby has made any improvements to the mechanism, but the originals ran well enough, and the detail is sweet.