So I just went a tried to shoot down Bretonia Train Shetland (130.000HP so slightly under SNAC damage) with Barghest equipped with EMP cannons.
It took whole 2 minutes to destroy it using shieldbusters. So on thrusters it would travel 18k before being blown up and only 9k before it looses 50% of hull.
So it is not like cannot scare it, even if you have EMP + SNAC loadout.