There is one forum member who put an A/C condenser on top of his stove and piped water through it.. he posted photos and his learning experiences..
Since he had the A/C condenser, and didn't want to buy the HilKoil SS water loop, he was money ahead...
He had leaks, and a noticable reduction of heat from the stove,,
The temperature inside the stove, just over the fire is 1500*, the average flue pipo temp is around 500* measured at the stove exit.. so the temps inside are considerably hotter than the outside..
Heat moves across a temperature differential.. so MUCH more heat flows across a ~1000*-to-55* temperature differential than will flow across a ~200*-to-55* differential.. even with much more area.
Your idea will work, but I'd not spend money on it,, if you have a spare automotive radiator or A/C condenser, give it a try.. but if you are paying for parts and supplies.. go with the inside the stove coil.. it will work best.