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I just can not understand why I am getting so much varied response from the piezos in my toms. I get one that works perfectly, one that is so loud that turning the gain down to 0 in MDM has no effect whatsoever, I have two toms that just produce a muffled sound despite turning the gain up to 8 in MDM and another that gives a medium volume. It's nothing to do with settings in Superior Drummer, they are all factory settings, haven't changed anything. I just don't get why the velocities vary so much. I have gone through everything, made sure the heads are all the same tension, triggers mounted properly, checked all my electrical connections. Done everything short of testing these piezos on a multimeter. Do values vary from piezo to piezo despite being sourced from the same manufacturer? Perplexed.

They do vary a little though not enough to match your description here.

How do you test piezos with a tester? You can really only test them with an oscilloscope.
Again, so much difference between your pads suggests there might be wiring problems with them.

Also pay attention to the consistency of mechanical attachment / decoupling dimensions and fixing, from one DIY trigger to the other(s)... I had one loud (roughly double signal strenght) RIM piezo on one of my modified T-Drum triggers (I added RIM piezos on the Tom-Tom triggers like the original Snare trigger had). Upon checking I discovered that for the loud piezo the double side tape used to fix the piezo element to the body of the trigger had a smaller (1 or 2 mm smaller) diameter than the ones that were ok...