I was looking around on Monkeywrenchracing the other day (thinking about getting an MR2 again) and noticed that stock 1ZZ-FE pistons are actually already ceramic coated from the factory!
Notice how the color above the rings is a little different. I wonder how common this is, and why some engines don't have it...
http://papers.sae.org/910460/
"At 1000 rpm, the 0.5 mm ceramic coated piston produced 10% higher thermal efficiency than the metal piston."
That's huge!
EDIT: Ah, this might be it:
http://www.sciencedirect.com/science...59431110004023
"Analysis results show that the surface temperature of the coated piston part was increased up to 100 °C"
Knock resistance could possibly be a problem with hot spots on the piston. That said, it seems like you can just use higher octane or a higher % ethanol blend and get more power if you want.