I suspect technically there is a HUGE difference between DCI's to Speedblades in terms of playability. There certainly was a noticable difference, apart from loft, when I changed my AP2's to X2Hot last season and DCI's are many sets earlier with one of the smallest sweetspots ive ever known. I can hit an X2 5 iron with roll out, in the summer 203 yards, my 4 iron in AP2 didn't go more than about 185, the X2 is easy to hit too - I agree that it's partly down to loft and shaft length but a lot more to better technology too IMO
Eh! Titleist DCI's are cavity backed, perimeter weighted, semi-game improvement irons. They are easy to hit, especially the long irons because they actually have a bit of loft.