I can confirm that the replacement powerball I order some years ago to replace my original did indeed play different. I'm still sad to this very day that I lost my original.
Lost it? Yeash, it was *probably* thrown into the trashcan by my friend's two-year old, who has a knack for tossing stuff in the bin. The trash was emptied before we found out the ball was missing (No, the kid is not the world's youngest pinball technician, this event happened during a tech troubleshooting week when the balls were laying on the table and he probably got hold of the nice shiny white one).
Anyways, back on topic. I'm not sure all after market replacements are equal, but the one I got (which was quite expensive as I recall) did play different - my feeling is that it plays "lighter". Snif.