I'm not sure what the material used is. I wonder how accurate the machining process is though? A factory wrx/STi uses an a and b piston because the bores aren't 100% accurate. I know the voodoo is a hand built motor but hand built probably means install parts out of a box.
I'm all too familiar with the ej ringlands. They will not tolerate the slightest detonation. I would still take a factory forged ej207 over any USDM mill though.
All forged pistons do this. Its because forged pistons expand and contract more than the standard hypereutectic pistons. If a forged slug was fit tightly into a bore it would scar the shit out of the walls when heated.