If you ask me, this is a loaded question. There is no way any one can say for sure how much boost your engine can handle. HRC claims that a stock engine can handle 8-10psi, but that is a best case scenario. What I mean is this, you may have an engine that has a rod bolt or a bearing that's just waiting to let go. Now if you dump boost to it then it's gonna let go faster. So you think it was the boost when actually it was bound to happen anyway. I hear stories of people blowing stock engines all the time. Basically what I'm trying to say is someone can tell you what boost they ran, but that doesn't mean your engine will perform the same way. I ran 13psi on a stock bottom end (I had a thicker head gasket to lower the compression but everything else was stock). I didn't have any probs, but that doesn't mean if I would have kept it up I wouldn't have eventually blown it. The bottom line is if your gonna run your car hard (turbo, NOS) then you should be prepared for the worst case scenario...blown engine. I would go with HRC's suggestion and not go over 10psi.
As far as you seeing any more power from pistons & rods the answer is no, not directly. The extra power comes from the ability to safely run more boost. Your building strength not power with a bottom end. All the power is made in the top end (intake, head, induction). Just my $.02. I didn't mean to come across like an ass if I was.
Joe
97 GS w/ Hahn stage 2
Keep cruisin' & keep the shiny side up.
http://joeseclipse.homestead.com