I haven't had any issue with Staff Shell at extended ranges. Only issue I had was with air vehicles, but its not intended for them anyway.
You don't need to engage enemy infantry if your tank's sole role is anti-vehicle.
It sounds odd, but its better for you to run away from infantry and staff shell enemy vehicles with cheap shots as much as possible. If you can single handedly take out enemy armour division, then just being able to pick off one or two infantry extra (whether its done by gunner or by yourself) is more than enough to make the trade off. Heck, just taking out one enemy armour with 100% certainty and just running away will net enough result in a big picture. Afterall, its 1-0 (ish, as ticket counting seems slightly different now) in ticket count and you are still alive to go and cap another flag.
Just by removing enemy armour, your team has a shield that is alive and funcitonal (and still deadly enough with gunner alone, or direct/indirect shell shot), while enemy has to deal with two different target classes and having higher risk of death.
HE shell takes 1 second longer to reload (not replenish), which can serve to be detrimental when facing against another armour that has active protection. And you have to work with more guessworks that lead to misses. I only find it to be useful in taking out IFV, as its two shot kill to the side. I used to use HE shell exclusively, but I have switched due to inconsistency in my performance.