Well Mercury offered a choice of the XR or #6 on the HP 575 SCI 550 HP motor. The good news about the dry sump in the #6 obviously is that it requires less oil in the sump reduces friction, adding 50 to 80 prop-shaft horsepower for improved overall boat performance. You can use an XR but you will loose around 100-160 HP on a twin application. That is a HUGE loss. The # 6's swept-back Dry-Sump Six drive improves handling and overall boat performance at higher speeds. The innovative "dry-sump" lubrication system features a precision-oiling pump that sprays a metered portion of oil to lubricate vital internal components. I think personally that anything over 500 HP needs the #6's. I wouldn't buy a boat without them if you plan on going to 550 HP and up.
525 EFI are fine with XR's. #6's for everything above. Plus the 6's look so cool and are so very efficient. Why loose 10% per engine at the prop with XR's if you have the $$ for a 550+ HP engine??