I don't mind the rangefinder bringing up the distance between yourself and the target. If it brings up the target hull and armour health bars up as well, that would be sweet.
I don't find it a problem for the rangefinder to highlight a projectile path of a given weapon the shooter is operating.... as long as it only shows the path that normal ammo would take, doesn't calculate the leading required to hit a moving targeted, nor calculate for the Coriolis effect if your ship is moving. If a ghost projectile path is too powerful, what about a ghost projectile instead? The spotter would have to keep a continuous bead on the target for the gunner to see the path that the ghost projectile takes, otherwise it disappears and the spotter can't go off to repair.
Because it would simply be bullshit if this tool allow newbie gunners to hit a squid going at full throttle from 1500 meters away while their own ship is being rocked about from minotaur fire. You want to make this tool more useful, not overpower it. Maybe give the rangefinder a minimum zoom level where it would discourage or at least make impractical to use at close range. Ie powerful binoculars to read a sign that is only ten meters away