i wouldn't say the ability is op per se, just very strong under the right circumstances.
IMHO the problem isn't the damage output itself, but rather how it is distributed between multiple targets. the reason is that number of shells fired doesn't depend on how many tanks are within the target circle, which means if you have multiple tanks in the area, the damage dealt per unit is rather low. OTOH, a single crippled tank will be focused down very quickly, making the AT overwatch really strong in taking out isolated units.
the easiest way to strike a better balance between single and multi-target performance would probably be to tone down the base DPS a bit, but let the number of shells or ROF scale with the number of targets in the aiming circle (maybe by a factor of 0.5 to 0.75). that way the AT overwatch wouldn't be an automatic death sentence for, say, a lone tiger with engine crit and do more than just a tickle when more units are around.
Wouldn't this make it very weird if the ability starts with 5 tanks in-range but after 1 shell drops 4 tanks leave the circle and halfway through the ability those 4 tanks return.
to make the ability more consistent/predictable it could be set to fire a set amount of shells per tank say; 5 shots of 120-240 dmg with 1000 pen. during a 30 seconds window while the ability duration is 90-120 seconds