Apexi “probably” has a design that allows for the S14 SR passage, and on the S13 obviously just sits against bare metal. I mean the passage is probably there, and probably just useless on the S13 SR, where there is no extra passage. Just a guess.
As far as bore and thickness is concerned:
Stock pistons are 86mm, Apexi uses 87mm for this.
They probably have a larger bore for larger pistons? Not sure.
Thickness will fully be dependent on how much (if any) material is decked off the block/head during a rebuild. I’m not sure on stock thickness right now, but you basically go with a greater thickness to make up for the lost material on the deck. Lots of companies offer a lot of thicknesses, and it’s basically what the builder wants at the time.