Interesting thread. I'm a first-time piano store owner (just received my first new pianos in May, 2014), and carry Yamaha among others. Years ago as a technician we used Windex and water 50/50 at a Yamaha dealer in San Jose, California. Now I use Cory.
Cory fills in the micro scratches, as it's designed to work with these finishes. I doubt Windex would hurt a polyester finish, but now that I own and owe $$$$$$ on my inventory, I don't mess with ammonia.
Buffing with McGuire's automotive polishes is legit, but knowing how to use a buffer is the key. I'd also recommend DIamond Glaze from Allied Nation Industries.
Finally, hire a great polyester person to detail your piano if need be, and have them show you how to take care of this finish. It shouldn't be this hard, and once you're on board, maintenance will be much easier.
Vince Chambers
Tenor
Owner - Apollo Piano
Chico, CA