A 0 offset hanger places the trigger and sear in a “default/standard” position, +.015 or -.015 moves the trigger forward or backwards .015”
On an action with ZERO cock on close, timing the trigger hanger can have an effect on the feel of cocking due to perfect timing of cocking piece to sear handoff, but on an Impact, which has a decent little bit of cock on close, I haven't felt any difference with the .015 hanger.
Essentially, in my opinion, you won’t be able to feel any difference in most cases, so you are pretty much adding or taking away .015” firing pin fall.