Reduce the load before chasing a rating
Replacing a functioning system can reduce energy use if the new equipment is materially more efficient, but energy savings alone must recover a large installed capital cost before the decision produces a financial return.
The comparison should use the existing system's actual condition and efficiency, the realistic replacement alternative, local energy prices and expected remaining ownership period.
Replacement may still make sense for reliability, comfort, refrigerant, noise or other reasons even when energy payback alone is weak.