[Piotrsko]

Well to get a heat sink that works as well as a radiator, you need the surface area of that radiator. It's a 3d radiator. You gain efficiency with higher coolant temps, higher pressure, slower speed of the cooling fluid, more air flow, maybe some other variables that would alter the delta involved.
You ARE sinking what ever btu you are burning generating horsepower, throughout the engine bay. Some leaves easily through the exhaust sysyem but that has losses also.

Somebody was once goofing with a Peltier exhaust system. Did work, but ROI was minimal

Nasa has an interest for their stealth aircraft since its a huge issue with targeting the aircraft you might not have the clearance to ask.