There are no upgrades for the 2016 7.4 (other than the Firebrand color). I am still on the fence between the 2015 7.3 Lime Green and the 2016 7.4 Firebrand (bright orange). I like both colors more than Trek's default black, gray or white.
I don't know if I can justify the cost difference for a carbon fork and slightly better derailleur.
I would like to add that of all the LBSs I've visited, the staff at the Trek Superstore were by far the most knowledgeable and helpful. (REI came in second and everyone else was useless. Avoid Dick's.)