The boiler loop sensor is on the boiler loop, between the radiant return and the boiler inlet. It's the only sensor that was in the proper location! (out of three).
You are correct about the pump ramping up and down, this was corrected by a plumber a few years ago by installing a manual valve in the radiant return line. If we put the valve at about 48%, the injection pump runs around 70%, and the boiler runs constantly until the supply loop is brought up to temp.
Put the valve at 100%, the boiler still runs constantly, but the injection pump spins up and down and up and down - never really get a consistant temperature out of it.
Putting it below 40% is a bad idea, the boiler starts to short cycle again (It hits its 180 F high limit and shuts off over and over).
What's the WM Ultra? I *think* the WM CGb Gold we have in the other end of the building is a condensing boiler, I like that thing. Never any issues, no rust, no complaining... Not very powerful though, have to supplement that one with the Trane's furnace when it gets below about 10. Just low BTU output, no mechanical problems (150,000 BTU).