I just wish the tubing I have in the sump was titanium instead of plastic tubing
I'm guessing it doesn't transfer heat very well.
Found
this to answer your question about mixing hot/cold water.
"Membrane performance is based on feedwater temperature of 25°C. For every 1°C below 25°C product water quantity is reduced by 3%. When feedwater temperature is regularly below 25°C, it is recommended that hot and cold water are mixed to increase the temperature to 25°C. Feedwater which is greater than 35° will damage most membranes. "
Just guessing that the input water temp from the cold water supply line would hover around 50F in the winter means that 50F= 10C. 25C- 10C= 15C difference from optimal temp. 15C x 3% reduction in product water per degree = 45% reduction in output water
To me it seems that it may pay off to run some hot and some cold in your case.