The twin 550's are a blast to fly no doubt, but the Airscan STC was for a specific mission. Apparently, that mission did not include carrying more than just the pilot at full tanks. They added plenty of power and some weight, but didn't ever get the gross weight increase.
Jack Riley was working on a GW increase STC to 5500lbs when he died. Increasing the GW to 5500lbs would've really made the 550 conversion attractive, but it never happened.
My guess is owners routinely fly them over gross weight with no ill effects whatsoever (except possibly an insurance issue or FAA violation).
FYI - O2A Skymasters have loading charts that go over 5500 lbs. And this is while performing low-level combat maneuvers.
