The history of M10001 shows the transition pretty clearly.
It started out with a 900HP V-12 and was rebuilt almost immediately with a 1200HP V-16, as the reality of rolling resistance and the minimum size of train dawned on the operator.
M10002 was the first of the articulated two unit power sets and added M10001's old V-12 to a V-16 to provide a real transcontinental train, if still rather tubular. Otto Perry has a photo of M-10002 running with a 1200HP booster from the M10004 series. This might have been an early trial before the bigger trains appeared. Of course the M10004 series became triples after E units appeared to haul even bigger trains.
But it emphasises W.A. harriman's statement about the trains being prototypes for further development, an attitude that seemed to get lost in the PR hype reproduced (quite faithfully) in Kratville's "Streamliners" (which I wouldn't do without once I saw it, to be fair).
Peter