there were some reinforcements to the stock and the polymer under the rail is a slightly different profile, the handguard now has vent holes, and a few other things I can't recall.
as far as what grain ammo - it should shoot better with the heavier ammo. The best shots I've made are with Silver State MK262 clone ammo, 77gr, privi 69 gr match is also very nice.
you won't see much with the dual optic unless you are shooting under 50 meters for precision. that sight is best used in a minute of pie plate role.