That was a problem because it was a dual purpose munition. The suggestions here is for a AT only munition with range 40 or bellow lower ROF and relatively low DPS allowing the unit to fire back at vehicles since the current projectile is not rather useless. Even the scott can hit and damage vehicles. The alternative is to increase the speed of projectile/lower trajectory so it can actually track vehicles.
Vet 1 ability is useless and should be replaced. Actually the direct fire could be the vet 1 ability, or the MG upgrade or some other type of munition or a barrage or an ability similar to kv-2 locking the unit in place but offering some bonuses.
The ability is not useless, even if it's the worst of the stun shots. I can recall enough situations where it saved a retreating squad from a light vehicle or turned the tide against a rushing vehicle. If you've got multiple Stugs and lots of munitions to spare, you can even have them take turns firing stun shots to keep an enemy vehicle ineffective and allow your AT units to fire on it unharmed.
I still feel the only thing the Stug E needs is a modest buff to its AI. Because of the tech lock, its timing is not far off from arguably better vehicles like the Ostwind and Scott now, while it should be a viable option as a doctrinal unit.