Here's a question why are the ram air doors not left open at cruise allowing more cold air in to cool and reducing the demand on the compressor/turbine then?
The ACM is not required in the cruise. In many aircraft there is a Turbine Bypass Valve that diverts all the air past it so it stops rotating. The heat exchangers function very well on their own, and the intake and exhaust flaps modulate shut as well as these heat exchangers are much too much at altitude.