I believe there is math for this very question. A similar principle applies with heatsinks. You cant just continue increasing the heatsink on a CPU, the outer e

I believe there is math for this very question. A similar principle applies with heatsinks. You cant just continue increasing the heatsink on a CPU, the outer edge of a large heatsink won't go above ambient and thus any heatsink bigger than that is wasting material. I would guess in a system where coolant is pumped and the added heat of that you'll have a similar problem. This is probably further exacerbated by the fact that you cant do clever things to increase surface area - your radiating surfaces must all "see" the black of space in order to function.