2.25mm to be exact(difference between both strokes) but can be remedied easily with rods, resulting in a nice boost in compression. I only mentioned that because he said he had a deal on pistons/cylinders/crank, IIRC.I thought the pin/dome heights were different between 916 and 853 pistons due to stroke diff? A 916 piston would be 2mm too low for good squish, right?
Also could be balance issues with weight of the 916 slug vs. 748. Better to go with the Pistals that have wrist pins balanced to stock 748 piston mass.
Balance wise, yes, I did mention there could be an issue, nothing that couldn't be fixed though.
One reason why I wondered who did the work. If you are mechanically inclined, it would not be an issue to remedy the issues. Also if you ended up getting a smoking deal on the parts.