If the injector spray pattern/atomization is messed up, I can see the results being low power and poor fuel economy due to poor combustion - it kinda makes sense. But then I would also expect there to be an observable difference in the way the engine runs - either idle noise, or perhaps additional black smoke under acceleration.

Unfortunately the piezo injectors are very expensive (~$500 each), so swapping them on speculation isn't feasible. I wonder if there are services which can test/evaluate/recondition them?