If you get the same EXACT piston (part number) you don't need to re-balance..unless the tops or pads were milled. Also, if you expect the rings to seat you will need to hone the affected cylinders. BTW, I had to do what you're about to do, but only 1 cylinder. (some a-hole dropped a BB down my carb....jealous ex-employee).
BTW, if you're still not using an inter-cooler, detonation is likely the reason. I can't hear any on mine...but reading the spark plugs told me I had occurrences of detonation.