550 ECU chip upgrade

It is usually on turbocharged cars where you can get a serious increase in HP from a chip upgrade, achieved by upping the boost pressure. I would doubt there is much improvement to be had over the factory chip on a 550 if you don't change anything else (like cams, cylinder head work, exhaust etc.)

 

Maybe you don't need an ECU. My check engine light was cleared up with new oxygen sensors.