So can I use my Hornady 165's and 180's without re-sighting since my muzzle velocity on both is 2700? (not target work...just plugging elk at...usually less than 100 yards but could be out to 250 yards)
Possibly due to many factors different loads will have different points of impact IE the 180s will probably shoot a little higher than 165s but this is not a hard and fast rules that all guns will follow.
you'll have to shoot both loads and verify POA vs POI at a couple different ranges then you can use tables to guess other ranges. there's really no shortcut.