During my research, in buying an ej255 or ej257 what I found is the piston P/N is different, probably accounting for the CR difference, but casting #'s for the block are the same (rod and crank are same part #, I recieved this info from a suby engine builder, name withheld, but he is a quality builder that make some top end motors, and many guys run his built motors). Also during my research I found that 2004-6 sti, forester xt, 2006-7 wrx heads all use the same casting part number, differences have to do with valves and cams, and machining work (ie. the air pump for 2007 sti, 06-07 wrx), after casting though the machine work may be different and the WRX head may have more stock machined from the cylinder head deck, also accounting for a higher CR. I need a sti head to take the measurements to compare. I can this evening however take a measure of the ej255 piston, and ej257 piston and tell you exact measurement differences, if it helps.
Note: This is not the bible, just what I've come up with during my blow up/re-build experience, I did work in a die-cast machine shop auto parts and cast # can be different yet it is the same part.
I can tell you, I have my ej255 open and ej257 right next to each other, that the crank is better machined (on the ej257) but as a machinist myself I can tell you that it would depend on the forging I received, and then the machine work (amount of stock) that was needed to be done (and let's not forget tooling, if I just changed insets in my fly cutter etc. etc.).
You can definately roll with the EJ255 for the build, but look out for CR with stock heads, perhaps an investment in a thicker head gasket is in order. I plan on doing a similar build that you are, with my blown block (255) and I have a set of 2002 wrx heads, after I finish my ej257 block build. If your worried about it I have EJ257 pistons (brand new) you can put in. If not if you run with the stock ej255 pistons and stock head gasket, just make sure your tuning is good.