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Count me in as a skeptic on the heat-from-thrust question. Could be, other aspects come into play, such as drag reduction because the rear flap on the P51 radiator nacelle may act to prevent backwash from the rapidly constricted shape, and/or the hot exiting air attenuates the drag from that shape. Maybe "pressure thrust" per Goldschmied/Bushnell at NASA is at play here. One way or another, apparently the thing gets less drag, which is tantamount to an increase in thrust.
For a good, recent application of careful thought and design of a heat vent into the slipstream, Google for the Arnold AR-5 and note the configuration of the forward right fuselage wall where engine heat is vented. Also, have a look at the various aspects of the Arnold AR-6 racing plane, especially the cooling inlet at the front of the cowling, which is designed to let max ram air in during climb (when the engine gets hottest) but more streamlined in cruise (when the angle of incidence is less, for less air entering).
For purposes of efficient venting of engine compartment air on a ground vehicle, I suggest starting with a horizontal application of the side vent shown on the Arnold AR-5. Perhaps you could have one sheet of Coroplast from the front bumper on back to near the firewall, then an exit gap where a second sheet curves down to and meets the same horizontal plane as the front sheet. Or, perhaps the rear sheet curves down to a parallel horizontal plane perhaps an inch or so higher than the horizontal plane of the front sheet. This way, the slipstream acts to suck air out of the engine bay per Bernouli and/or Venturi effect(s).
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