Agreed, but the initial modelling wouldn't have given that result unless the installed power was comparable (so one Flex engine raft is worth two Sprinter engines).
Surely it isn't just about the absolute power the installed engine can give. There's differences in overall unit weight, (nearly 160t for a 769 vs about 144t for a 150), then there's the gearing of the units, 769 is for 100mph vs 150 for 75mph. Those two both weigh against the 769s, but the torque/output speed profile of a diesel engine/generator/DC motor is far more flexible than that of a diesel engine/torque converter, so there are reasons to believe that even in a stopper role, the 769 may be no worse than a 150 and probably a lot kinder to the engine in service.
One thing that has so far not been mentioned here is that when a 150 (2-car) has a failed engine, not only is the tractive effort halved but so is the adhesion. On the 769's case, although if 319 experience is to go by, motors do get disconnected for various reasons, they are generally getting an easier life in the north-west so that is less likely, however if an engine fails, performance would of course hardly be stellar but adhesion would not change at all so a limping home train is unlikely to completely stall on difficult gradients/railhead conditions, (unless the 750VDC bus is split between pairs of motors when running on diesel).