It WAS the wrong shunt resistor. On removal and inspecction, it was clearly stamped on the underside, "100a 75mv". Oops. I found a correct 100a 50mv shunt in my electrical junk, and switched them out. So, while I probably had increased current for a given speed in the last round of tests due to voltage drops across iffy connections, this increased current was also apparently overreported by the ammeter, by 50%! So this goes far in explaining why current/speed was so high, and why my 10hr, 10a dock trial left extra juice in the bank. I was actually running at about 6.7a while the meter was reading 10a, expending a total of 67ah instead of 100ah. with expected Peukert and other losses of around 7% at such a gentle discharge rate, that works out about right. So still, the batteries are performing at pretty close to new factory fresh levels. I won't bother repeating that test again for another year.
Looks like thunderstorms today so even when the batts are topped up I won't be able to speed test. Hopefully first thing in the morning. With this discovery and correction in my instrumentation, and hopefully rigging a 20 point light so I have legal navigation lights, it will still have been a productive day. Maybe I will borrow a windohs computerr from somebody (I run linux and the configuration utility for tthe controller only runs in windoze.) I will make sure that the max battery current and motor current are where they need to be. I didn't think about that before but the DC into the controller should be about 1.43x the motor current, and that may be part of the reason I never got the expected power level out of the old controller. It was limited by the max battery current setting, I think, which I probably set equal to motor current limit. Documentation for these Kelly controllers is not what it could be.
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