A Starhawks payload would also make short work of any ground tank, so that's a moot point. And furthermore, in terms of this gun, it's still only shooting ONE shot, that it eventually rams into. So it's not making anywhere near the damage saturation a Starhawk can produce.
In game terms, the Caestus operates almost exactly like a boarding torp, ramming into the opponent and off-loading a small team of armed Marines that cause havoc inside a defensless ship. From everything I've seen, the Caestus looks about this size of a Fury Intercepter, which from what I understand should be smaller than a standard boarding torpedo, so therefore would have the same overall effectiveness. The Shark (or T'Hawk) lodges onto a ship, and off-loads 3-4 times the amount of Marines. So the Shark loses the element of surprise, but has the numbers to make up for it.
The Caestus is an A-boat that should attack ships like boarding torps. If you want to go crazy, you can give them 2 Caestus for every point of Launch Bay, or give it 35-40cm speed, or give it resiliency. But that's all this little guy is going to get.
-Zhukov