For the extra $400 you got a couple of XL padeyes and a slightly better seat strap.. Maybe $30 or so in accessories. I'm guessing the 2015 was a run out model sold cheap? Does he have one on show and if so which version. First one I got in late april this year was a palmed off early 2015 without being advised it was superseded. Took on water with or without gooping, so had it replaced under warranty.
On 2016 the thru hull plugs in the side pockets at the front are also on side, not at the bottom towards the rear, again the using surface mounted 3 ways rather than 2 way style with backnut
Changes listed here;
http://www.hobiecat.com.au/mirage/mirage-revolution-16/specs/though not obvious what it all means until you see them
A please explain from your dealer is required, get him to point out the differences and explain why you haven't got them.
Dont get me wrong, love the correctly fitted 2016 model, $400 off the old model and having that hole welded up would still be worth it, as its not a big job for someone who's competent at it. It may be something your dealer is capable of doing.