A closed loop system would be great as long as the range of adjustment allowed by the ECU and the various sensors was broad enough to be useful for performance gains, not just limited to emissions. Most stock closed loop systems are intentionally limited in their range of compensation/adjustment to satisfy the feds. On your late model car for example, the range of adjustment allowed by the factory mapping is not very wide. If the ECU sees a reading out of range and is unable to compensate for it within it's predetermined mapped range of adjustment it will illuminate the "check engine" light. Aftermarket mods allow the ECU to make performance adjustments beyond the factory limits by either increasing the limits or offsetting the values of sensors to fool the ECU into thinking the values are still within limits when in fact they are not. Closed loop systems allow ECU's to do this adjusting "on the fly", better compensating for changes in operating conditions rather than relying on a fixed map that while it works fairly well, relys pretty much on predetermined settings, just better settings than stock.