Pistons aren't the only factor in octane requirements. Cam choice also plays a large part in this (if you're considering a cam swap).
If you have to bore the cylinders, you have to get new pistons anyway. Unless you're buying some custom units, I don't see where something with a little more compression is going to add much (if any) to your build. Add the fact that worse case scenario, you have to deck the block or mill the heads to achieve a flat surface... You may end up with more than you want anyhow.
If you're only going to use this for what you're saying, I can't see how it's worth putting a lot of time/effort into something so you can run fuel that's $.10 cheaper a gallon?