The 321s problem isn't lack of power, it's lack of lift. All the 320 series use the same wing (span - the flaps are different on the 321) and it makes the 321 struggle at altitude. The 321 wasn't designed as a competitor to the 757, it was intended to be a higher density 320. It just fitted into the Market slot occupied by the 757.
And pilots tend to forget that the real reason why aircraft fly is not the laws of aerodynamics, but the laws of economics which is why the 321 is still in production and the 757 isn't.