Okay, The piston slap is common even in phase II block and I wouldn't say it's so much a problem just something that is subaru. Here is a quote borrowed from Wac talking about the new upgraded gasket for the DOHC: Subaru has since replaced this design with a newer one. Although I don't have a picture, I can tell you that the newer design replaces the outermost graphite layers with solid steel. The fire ring is also much more heavily reinforced with steel than before. Check out his website
Here For info on headgasket replacement and such.
Joshua