You can send the block to outfront motorsports and they will close deck it for $800 dollars. If you are not looking for more than 400hp You can go with STI rods but I would run forged pistons. The USDM oem Subaru pistons are junk. I have rebuilt at least 12 motors because the piston ringlands cracked.
The only other difference in the blocks besides the semi closed is the EJ257 has an extra crankcase breather in the middle top of the right block half. You can tap the NA block and put a breather there. I have done it before on a EJ205 that I put a STI crank into.