The track represents the largest change, that being said, There were also a lot of little changes which IMO represent very valuable improvements.
The LT track upgrade is $900 with the drop brackets for a 2015 LT kit.
Basically the whole rear suspension was changed for the '16 kits. the long front arm, new jounce bumpers, and scissor rear arm are all new. Also the new splined upper shaft, lighter gears, and threaded shaft instead of drilled and tapped. Lots of little changes that "could" be upgraded but would not make sense dollar wise. The upgrades of the rear suspension make for way better handling and much more plush ride compared to previous kits are
Im curious what the '15 LT costs, because by the time you throw 900 bones at it, the 16 kit still has all the little upgrades. Point being to me, unless your tight for dollars, I would be hard pressed to get a 15 over the 16, just so many improvements.