Slight raise in hp and compression, minimal.
What you WANT to do is get oversized pistons so you can assure yourself that you will recieve a proper seal with your rings. You have the machine shop bore to piston size (.25 over or whatever you choose). This way you can be assured everything will work perfectly, and you wont end up with leaky rings, or smokey exhaust.

On a side note, I realize there isn't anything wrong with an LS rod/ITR piston combo. I just would think if your planning to beat this motor, eagle rods and a fitted Endyne Wiseco piston combo would be a better investment. Granted that's 800 bucks, but it's a WELL spent 800 dollars.

Just my opinion.