What does it mean when the piston is .1 or .2 over? Does this mean the piston is larger!

yes. The .1 or .2 refers to how much larger than stock they are. This is the bore measurement, not stroke.

I would have to bore out the engine then right? How much does it cost to do that?

Not much. Depends on the machine shop that you take the block to.

If you have the tools, you can do it yourself.

What power gains would this give, if they are also 12.5 compression?

Power gains are very small if there is any at all. Sometimes the cylinders are not round anymore or scarred or whatever. So when they are rebuilding the motor they overbore the cylinders either .1 or .2mm over stock so they are returned to being round. Most aftermarket pistons are made this way because most people who rebuild their engines do this so that the cylniders are rounded again. Of course they do sell stock bore if you don't want to overbore the cylinders.

I thought adding compression gave some good HP? Well in N/T terms!

It does, but as far as the overbore is concerned, it does nothing as it does not increase displacement signifigantly. Also, just for the record, it's usually a .020 overbore (twenty thousanths of one inch).

HIgher compression is good for turbo also, hehe you'll detonate more at a certain psi than with lower compression though.

Actually, if everything is set up right, the odds of detonation happing are about the same as with the lower compression pistons, teh problem is that detonation with higher comp pistons can result in more damage than with the lower comp pistons. If you are going higher compression with boost, make sure you use racing fuel or alot of octane booster.