Heavy Ballista is definitely powerful (possibly OP- but I try to avoid phrases that can induce flame wars). It is easy to trigger an increase in its base attack and it isn't particularly hard to end up with an attack value of 8- especially with cards like Wulven Tinker that increase its durability.

In fact, its ability is easier to trigger than the 6cc Chains of Torment which requires an opposing ally to be killed and only offers 2 more durability for twice the cost. And again, the durability difference seems to be further trivialized by the presence of Wulven Tinker.
(Mournblade is another potential card for comparison but because it is designed for a different faction with access to Tidal Wave- it doesn't seem to be a fair comparison.)

In addition, Heavy Ballista increases the attack value of your (high cost) allies- acting like a War Banner or a Meat Wagon.
All in all, based on the existing cards Heavy Ballista definitely seems like a power creep compared to Chains of Torment without offering any balancing drawbacks.

Hopefully this post will trigger constructive discussion as to what changes would be necessary to make Heavy Ballista not a power creep.