Tinker I don't think a 15deg iron and a 15deg fairway wood would perform the same - length is a factor, but I imagine the design of the head plays some kind of role.
Overall though, yes, this vanishing loft stuff is bullshit. "Omg I added 12yds per club with my new Callaway set, but I can't hit the 5i, and I use the 4i to putt! Who cares, I'm hitting the 8i 210yds!"
The Sterling irons actually have wild lofts. You need to totally recalibrate what you think a 6 7 8 9i distance is. It's not that these irons are magic - they're just different. This 6i is not meant to be a direct replacement to your previous 6i. This 6i goes a little lower and a little farther than this 7i, and a little higher and shorter than this 5i.
I love the Sterlings, BTW. They're very honest about the 4i and 5i jot being viable for many. They don't even make a 3i.