A couple of thoughts from me

My knowledge relates to research done for a heat pump here in UK

Those summer/winter temperature figures surprise me as I would expect ground temps at 6-10 ft deep to fluctuate less seasonally when the heat load going in is somewhat constant.

If you are thinking of connecting to the same loop as your air-con then I understand where he is getting those figures... I am not sure that is such a great idea as you would effectively be cooling with warmer water produced by your hvac.... unless of course you can oversize that loop. However, the pressure issue could be overcome by using a plate heat exchanger thereby allowing you to run your rig-side loop at whatever flow/pressure you wish.

Personally, I would dig for a second geo loop at the same time as I had the machine digging the hvac system I think.