PA-PLAYA I don't think there's a lot of variance in the entire PING G-series irons. That's what I've come to enjoy about PING. They don't make radical changes and the G15's probably don't play all that much different from the current G's I have.
They don't try to reinvent the wheel. If it works, no need to fix it. Most of the discernible difference is in the cosmetics and feel of the ball coming off the face. They all pretty much perform the same. Oversized, forgiving, designed to get the ball in the air a little sooner.
Interesting observation. PING did change the G series starting with the G25 model by reducing the sole and topline width and welding on thin springy faces. They also started jacking the lofts with that model. They changed the finish starting with the G and changed it again with the G400. All that being said, they STILL perform about the same - designed to be forgiving and hit high, straight shots, the only difference being distance. Owners of G15 would possibly be using a 9i, where G400 users might be hitting W, but YMMV!
Regarding the finish - my G400 irons are going to be pretty marked up in a few months. I can see it coming and have read a lot of user feedback. The Hydropearl finish is not durable like the old Guyson/Tumbled irons. The good news is, that the Hydropearl finish and the face coating scrubs off with Barkeepers Friend and a Scotchbrite pad pretty easy to get down to the stainless, which shines pretty bright (as shown by others). I have heard that about 10 minutes scrubbing per club makes them like new again, and also takes the finish down to the durable level.