It'll be just as accurate, but will require some sort of key press input to function like a standard ability and may have some input delay (although significantly less than if you were to use mouse position). I would create a dummy ability that does nothing but has the same cost/cooldown as the regular ability to fire with "regular mode".
So use QuickCastModeOn, and give players that want to use regular old targeting mode an upgrade that replaces the ability buttons on their command card with (identical) buttons for dummy abilities.
As for how targeting mode works, take a look at the attached demo map. Pressing "1" enters targeting mode for psi storm for the selected templars, functionally mimicking how normal ability targeting works which can be used in conjunction with Quick Cast mode (press T to use psi storm this way).
ALTERNATIVELY, with you could just use QuickCastModeOnUp as a compromise since it shows the ability targeting actor while the ability hotkey is held down.