 I don't know where the 645 HP number came from; the old single-engine number I remember was similar to the HP of a small aircraft engine.  You omitted the M400 would have eight engines rather than the number of engines in the RX-8.  Only four engines would be needed for vertical landing (when flying with the aerodynamics of a brick is relevant), but all eight would usually be used at less than full power during vertical flight, when the dual redundancy in each engine pod is required.  The intent of the design is for a single engine in a pod, at full power, to be able to handle the load if one engine fails.  When in horizontal flight, full power from all engines would not normally be required.  Also, the Rotapower engines have an ancestry stemming from marine engines which may be somewhat different from Mazda's.  Comparison with anything else is indeed difficult.       