[Efthreeoh]

The HVAC system has zero to do with engine cooling capacity when both systems are in good* operational states. The blower fan in the HVAC will not affect the cooling ability of the engine cooling system. When not using the AC, the coolest air available is the ambient temperature of the outside air the car is travelling through. It will be a few degrees warmer though, because the HVAC plumbing (vent system) is under the dashboard and windshield and picks up radiant heat and stores it in the plastic, which transfers to the air flowing through the HVAC system. It is perfectly fine to drive the car with the HVAC system turned off. The hot air of the engine bay does not travel through the cabin HVAC system. Hot engine bay air flows out the bottom of the car. The HVAC system draws outside air for the cabin from in front of the windshield. There is a seal between the engine bay and the HVAC intake. On the E9X it is the rubber seal on the lower cabin airfilter tray that seals when the hood is closed.

* in the case where the engine is overheating or running very hot, using the heater can slightly reduce the coolant temperature as the heating core acts as a secondary heat exchanger (i.e. radiator) to bleed off a small amount of heat from the coolant. The passenger compartment gets a bit warm though in the summer.