Like a sports car (or any vehicle for that matter, I guess) evolves as model years pass, so has Titleist's Advanced Performance irons. The newer models might look pretty similar at first glance, and in some ways, they are, although there are loads of significant updates that shouldn't be overlooked. As you know, more weight here, less offset there, and some added horsepower under the hood can make a world of difference.
In 2008, Titleist broke their own mold a bit with the release of the AP1 and AP2 irons. Never before had the (in Alan's words) "traditionally traditional" Titleist released irons with more visible technology and with that they stirred quite a buzz. Fast forward to August of 2009 where we learned that Titleist had done some fine-tuning and enhancement to both the AP1 and AP2 irons. Luckily, I had the chance to give the Titleist 710 AP1 irons a spin.