There are two other variables that haven't been mentioned yet.
A tapered piston has less surface area around it's circumference to interact with the shock body as it gets thinner at the outside. Also on the tapered side the angle between that face and the edge of the piston is less than the 90 degrees of the flat side.
Would the lesser angle change pack differently in each direction? Or would the reduced area make the way the shock pack's more progressive (is the oil flow turbulent all the way round the side of the piston when it packs
)?