Yeah tracking a tank is rather useless, would take just as long to track a tank as taking up a Hurri-D and blowing his turret out or engine or both. Quite simple instead of aiming guns at tracks, climb high above the GV from behind, dive down on top, I use my guns at 450 converg and get usually 4-6 shots off before i pull out.
On general you will more likely turret the tank, if he stops suddenly you more likely shot his engine out either way you stopped him.
I kinda enjoy tracking tanks in middle of no where though, you see them run circles for 5-10 minutes before realizing nobody is running them supplies.
It does take ALOT of practice to aim those 40mm, they are pretty easy to aim, if you want to make sure you hit and kill something set your guns to 250converg
I've used 650 in the past but seem to only hit side of the tanks at 400 out so I settled with around 400-450