[jeff5may]

If heating is your main concern, the ground would be a better choice for the figures you quote. With the loop in the bottom of the pond, it will be exposed to the heaviest water in the pond. As you already stated, this water will always bottom out at 39 degF. Not great for performance. The experts state the 350' per ton is good only for cooling duty with a well exposed loop, flowing water or convection flow. For a stagnant fluid bed and heating duty, they recommend 400 to 500. If you were doing lots of cooling with the system, the pond would be a better fit.

Regarding the slinky loop, the longer and deeper the loop is, the better. Since the pipe is coiled, it isn't quite as effective as straight runs. Minimal oversizing pays off big time. The deeper the loop is laid in the ground, the less seasonal temperature swinging it will experience.