I initially wondered if the fin was in direct physical contact with the tubes, because if the solder/braze filler was bridging a gap that would be bad...filler materials are crappy heat conductors compared to copper and aluminum. BW assured me that was not the case.
The next big question mark is a surface area issue. Their flat tubes are unique, and all else being equal a flattened tube should be superior to a round one of similar circumference due to improved turbulence, but I don't know that they're using enough of them. I'd love to get my hands on one and take some measurements.
Their FPI now that they've upped it to 20 (from the original 11 to 15, and up again) is at least closer to the competition (which are generally in the mid 20s). Until I get a chance to actually measure their tubes and see how many of them there are, I can't eliminate that as a possibility. But I also don't know of anyone on the forum that has used an updated one so the issue may already be fixed.
They increase turbulence if the flow isn't already fast enough to achieve it. If the velocity is slow enough to be in laminar or early transitional flow you need the turbulators to make sure all of the fluid gets a chance to touch the inside of the tube. If the tube is flat like brassworks, they aren't needed (and not really possible, anyway). This is an example of how slowing down the flow is counterproductive to the heat exchanger performance, not helpful.
you were putting the BTUs into a smaller mass.
Q = m * Cp * dT
Q is the heat energy transferred in BTU or Joules
m is the mass
Cp is the specific heat capacity of the substance - how many BTU does it take to heat 1 pound by 1F degree
dT - the temperature change of the fluid.
This can be done for a fixed volume or you can add a time element by turning m into mass flow rate and Q into energy per unit time, aka power. (BTU/hr or Joules/second which is Watts)
By slowing down the flow, you reduced your m, and your dT went up. Your Q actually went down, because the mean temp difference between the hot side and cold side shrank due to the increase at the wort output. That temperature difference is a direct input to the math used to predict the performance of a given heat exchanger.
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