I didn't weigh my pistons before (
) but it's a very thin coating that partially penetrates. The anti-friction on the skirts adds somewhere from .0004 to .0008 (less than a thousandth), so no real changes needed to bore.
Piston skirts: $18.50 ea.
Piston top (gold ceramic): $29 ea.
Those are retail prices. They also have oil-shedding and anti-corrosive coatings.