.020 over on a 5.9 for 1000hp+

No issues with going .020 over on bore. Just like you said earlier its only .010 on each side. Plenty of meat still left. As far as pistons, I trust stock 03/04 pistons with even just the crowns coated. Definitely helps soak up the heat and keeps from cracking

I drilled a hole in a junk standard bore block I have at work. The oil cooler was removed and I drilled a hole through the number 2 cylinder, it was a stock bore and measured about a 1/4 inch of wall thickness, just throwing that out there.

The DLC coating on the skirts might be cheap insurance to keep the piston from scuffing the cylinder when it soaks up all that heat real fast.
The ceramic heat reflecting coating on the piston crowns does work, but its not a huge improvement. I have run both on gas bike motors that were used for high speed hooliganism, lol. These pistons have an ultra-short skirt often with oversquare bore/stroke. I think even the mfr's are using DLC to improve piston life.