Here is the poop. Stock oem pistons are offset to one side for quiet operation. Minimal gains of a couple to five hp can be had by flipping them. For a street build, it isnt worth it to me taking a chance on cracking a piston on pressed pins to flip them.
IIRC engine masters did the piston flip with aftermarket performace pistons most of which do not have the offset anyway. So nothing to gain by the flip.