By sag you mean compress? Then yes, under load all springs will compress. Some more than others and Teraflex, Rock Krawler, Synergy etc. list the lift height after compression under vehicle. Therefore you buy a 2.5” lift from RK, once installed it will be at 2.5” (usually a little higher). RC on the other hand is great brand new but over time your rig is lower than advertised height.