Hey EdDie,
I did this mostly as an experiment...
It's a lot of equipment and $$$ for what you get. Pelt chillers just aren't very efficient IMHO. You get a lot more bang (cooling) for your buck by mounting the Pelt onto the CPU (with a cold plate) and watercooling the hot side.
Yes, if I was to cool the hot side of the Pelts with chill water then the cold side would go well below freezing.
In final form, two auto heater core radiators with shrouds are a better choice (less expensive and greater surface area) than the two BIE rads I used.
As I'm sure you already know, copper is much better at conducting heat than aluminum - so "better" yes. Enough "better" to offset the cost and hassle of machining... can't answer that for you. That's the main reason I went with two commercial waterblocks for the Pelt sandwich.
Good luck,