The tension in the movie was centered around the question whether Ford really could build a race car to compete with Ferrari. Two of the key metrics were 1) can the engine handle producing that much power, at that high of an RPM, for that duration of time and 2) can Ford figure out how to design brakes that can handle the extreme stresses without significant fade or failure.

If the engine didn't hold together, they couldn't win the race. If the brakes failed, it would kill the driver (ultimately did a year later).

Horsepower is only one aspect... and MPH is nice icing on the cake, but is really only a secondary item. The overriding factors were engine longevity and brake function. The movie was correct to focus on those 2 items.