Having heard Runnicles conduct the Ring, Lohengrin, Flying Dutchman (twice), and, most magnificently, Tristan und Isolde (also twice), I'm happy to confirm that he's the real thing in Wagner. Last year's Tristan was beautiful beyond my ability to describe it, though ACD might not have liked the astonishing orchestral transparency, in which you could hear every line and instrument, if you listened, and it all added up to a magnificent whole.
He's less than perfectly happy with the singing.
However, Christine Brewer is a marvelous singer, with a big, gorgeous voice and stamina to spare. I'm looking forward to hearing her in the Ring, sooner or later.
Update: That big strikeout is because I saw Kathleen, not Katherine, Broderick at ENO. Thanks to AlisonC, who pointed this out to me in comments.