Oddly enough, the 2.5RS pistons look a little beefier then the 2.5STI ones.
The difference in length is normal, as I'm sure Ron knows. There is only about .5-1mm difference in length between all Subaru rods as they mainly move the wrist pin -to- piston crown height around to make them fit correctly.
The ribbing in the case is definitely interesting. I didn't think there was a problem there, but I guess they decided on a little more sturdy case as well.
The tolerance stamps look pretty much right on with everything else I've seen in the 2.5RS blocks. ABAB is really really common... stupid machinists, can't even get them all the same diameter. Only one I've seen that was perfect is the motor that is currently in my car, AAAA.
Would you mind taking a pic of the backside of a stock piston, as well?