well if they've finished the album, obviously the singer they are using has been decided. But DC is only confirmed on 1 song.
To be honest, I like him on "Paradox" the 1 album i enjoy from Royal Hunt, and he worked well on the stuff he has done with SG, but primarily as a character/secondary/complementary voice. As a lead singer in SG?..I dunno. It depends on the WRITING OF THE VOCAL LINES. As he has a good enough voice if the melodies are good, his voice works. But his voice is different than Mike's. he's more of an alto, and thus, the classic SG soprano lines may not work or be present on this new album, which just on the surface I'd likely not prefer.
This might in some ways remind me of when Nick D'Virgilio took over the lead vocals for Spock's Beard. Sure, technically he's a very good singer, but HIS VOICE doesn't give the same impact that Neal Morse's did. Definitely a case of a good complementary singer (at least for this kind of music) that really doesn't fit as well doing the lead vocals throughout an album.