If it's the clear flat acrylic I have heard nothing at all bad about it. A lot of people whose opinions I respect very much say it's the best clear flat going.
As to thinning it, if it's already thin enough to spray why thin it at all? The only reason to thin any kind of paint is to get it to spray at a given pressure. All of the PolyS paints I've used are plenty thin right from the bottle.