I believe 5 stacks of Instant-Tracer Missiles is a bit much.
But, I'm not against 5 stacks of using Tracer Missile while running. (Channels like normal, but allows movement during channel time).
It would do no more damage than 5 normally casted Tracer Missiles would. It simply allows Mercs to use Tracer Missiles while they are pressured by opponents.
Merc's largest weakness is being interrupted (not just by interrupts, buy by knockbacks/leaps/CC/etc), and a brief period of being able to use casted abilities instantly would give Merc a tool with which to fight back and try to win a fight, rather than just run away