I went the 4" angle grinder with a cut off disc and a jig I made out of a bit of 2x4 wood, made a hole that'll fit over the sanke tap with a hole saw, and a jig saw to cut out the end of the wood so I can slide the angle grinder down into it to make a perfect circle cut.
I had originally tried using my dremel, but it took forever and went through a half a dozen discs easily, and left me with something that only barely resembled a circle, same deal with the recip saw since it kept wanting to go straight, and free handing with the angle grinder and cut off disc resulted in similar results, though marginally better when I drew a circle with a sharpie with a piece of string tied to the end of it and knotted around the sanke tap.
For the lauter tun question, what in particular are you wondering about?