I am also using Teco RA680, had ask the same question and after all the input, decided to have the water taken from Sump and return back to sump.
but I think my sump are bigger than yours therefore, it still took 3-4hrs to chill from 29 to 27degrees.
I am using 1500l/hrs pump to the chiller, sump and chiller same height.
i would be interested if you can connect better and with better efficiency. Pls share with me.