$40 is a good deal as long as no one else grabs it at the last minute, that sucks.
As far as piston coatings go a lot of companies have been doing that, I work mainly on high performance cars and coatings were common on the skirts of aftermarket pistons. Then they started doing the tops as well.
Now a lot of vehicles come with coated pistons, even non high performance.
I'm not sure about coated motorcycle pistons, but for the car pistons the skirt clearance was set by honing the bore to the spec on the box, NOT by the traditional method of measuring the skirt, adding clearance specs, and honing it to that number.
The coating also seems to eliminate cold start piston noise.
I've been looking at the Namura coated pistons for the next on in my bike.