I really enjoy the B&W results I attain from my old D800e. There's something special about them, whether it's just the dynamic range or the color response or a combination of factors, I don't know. But converting to B&W with either Google/Nik software or OnOne Perfect Effects software gives me terrific results that I (and others - even my wife) can distinguish from other cameras I own, particularly my Canon 5D2 and 5D3. I also use a D810 and Sony A7II and A7R2 but I really haven't pitted the D800e against them yet when it comes to B&W. Even if they're as capable as (or better than) the D800e for B&W, a low priced, used D800e would rock as a dedicated B&W cam if one already has appropriate lenses.
As for the Sigma Merrills and B&W, I rented a DP2 with which to play and agree that it produces very, very nice B&W images. A friend here in Juneau who is a full time award winning pro photog and Nikon devotee keeps a DP2 around just for B&W work.
Cheers!