I don't think you'll like the riff raff
you said you wanted a half decent metal tone :(
the RR is certainly not "half decent" for anything :D
:lol:
Yep, I'd recommend Mule or Riff Raff too. I have both. I think of Riff Raff giving me a tighter tone. If I do venture towards metal (we're talking pre '83 or so here, that's as far as I go!), then the Riff Raff is what I reach for myself.
I also have Mississippi Queens (hot p90 in a humbucker size unit). I tend not to mix-and-match pickups so far, but I'm certain that an MQ neck would work well with either the Mule or Riff Raff bridge.
On the coil tap front, I wouldn't have thought it would affect the humbucker tone - or at least not so that any of us could hear it.