I think the risk of dust and debris getting in outweighs any advantage of letting heat out. Further, I think that heat from the HPWC is dissipated by conduction, not convection anyway.
If so, I suppose one (Tesla?) could fashion some sort of heat sink to aid in dissipation, if indeed the heat is detrimental to the lifespan on the unit.
Also, count me in as another user interested in Ingineer's 40% solution. I'm not seeing a 40% loss (more like 16% when comparing energy expended to energy delivered), but I'm assuming he means something different by that efficiency number. Definitely interested in slimming any losses en route to the car.