I did some more testing today looking at chilling performance while adding water to the condenser, and operating in cloudy, somewhat cooler. In all cases, lowering the pressure causes a huge loss in chilling. Reducing the charge made it worse.I only got back to near the 11K BTU by increasing charge back to high pressures (430+ high side, 132 low). Again, I watched cooling slowly improve as charge and pressure increased.
The other observation is that my previous 68F at the suction line at the evaporator was correct and is only achievable after long running; it starts about 74F. The evaporator suction line temperature right near the BPHE stays within a few degrees of water input temperature, so it will never be much lower than 68F in real use.
That concerns me about using a TXV, since they are designed for a window AC unit, with around 38-45F temperature at the evaporator suction line. That's a long, long way from 68-78F.
It also concerns me about evaporator performance; I'm wondering if the ONLY way to have evaporator performance is to reduce the superheat. In this case, I'll have no choice but to run at high pressure, as I am now. I think perhaps high superheat is OK in this case, since clearly I'm extracting all the available coolth of the refrigerant, and I just need more refrigerant flow and by shortening the cap tubes I can achieve that at a lower high side pressure.
I'd love the regulation of the TXV for better performance across a range of ambient temperatures, but unless they have one for much higher temperatures at the evaporator I'm concerned.
I may try shortening the cap tubes substantially and see how that works out. From charts I've seen, a 10 degree change of evaporator temperature makes a 20% change in length. I need about 30 degrees and 60% shorter....maybe. I'll study some charts more to see how linear it might be over a larger range.