OK i'm not a fanboy of OS & i too have concerns about their lifespan BUT are you guys sure the cranks on your engines were pushed all the way through the bearings? i know tightening the flywheel should ensure this but it can happen!
plus the only reason i can think of for the backplate getting marked is the crankpin wearing!
when the crankpins wear down you will find the rod slips sideways & rubs against the backplate due to the cone like shape a worn crankpin develops! there only needs to be a slight amount of wear on the pin for this to happen & once it starts it gets rapidly worse, especially once you introduce alloy particles from the backplate into the engine [:'(]

apparently the modified versions of the OS like the "Recab" have cured this problem by hardcoating the crank which (so i've been led to believe) really extends the engines life, the cranks are also bored out bigger & have the epoxy resin stuffer too