It would only be balanced if
1. Sufficiently long cool-down
2. Don't work on retreating units or has a pretty low success %
Why? Because it would lead to cheesy squad wipes where you have I&R pathfinders constantly picking off the last models on retreating squads.
Personally I like the Airborne Pathfinders as they are now - I'd much rather have a free passive ability that works some of the time and frees up Muni for those delicious Paratrooper upgrades.
The standard sniper chance to hit a retreating a model/s would apply. Although the Ost sniper's incendiary round also has a 100% hit rate on retreating models I believe? Albeit that is more expensive.
A 15 second cooldown (not including the time the ability is "active") would be sufficient imo.