Andrew,
I'm no expert but I do own a 1999 5.0GL/SX outdrive. The parts are more expensive and harder to find. Believe me I know, I just got my boat back. My outdrive locked up with only 70 hours on it and it took 2 months to get it repaired. I even had to raise HE** to get the parts shipped from Sweden to get it repaired that quick. The price to repair the outdrive was $4500.00!!!! Thank goodness for the extended warranty I purchased. It is supposed to be more user friendly as far as normal service. I'm not sure if the unit is more effecient but my 20 foot bowrider with this 220 horse motor runs 59 MPH with a full take of gas on board. If I had the chance to by it again I wouldn't, I would buy a Merc just for the price and parts issue. I may have just got bad service but parts are more available for a Merc.
Hope you weren't in love with this boat.
Robert.....