I know what you think of when you think of a CCP "repair". ^^

But I think this is unfortunate that this weapon is almost nonexistent on the battlefield.

Yes a buff splash could be significant but not a buff of the splash damage to prevent the weapon becomes op.

I firmly believe an Allotek Plasma Cannon should be able to 2HK a well fitted Gunnlogi. STD and ADV Cannons should take 3. A well fitted Maddy should take 3 Allotek rounds, or 4 non-Allotek rounds.
I don't believe this is unbalanced in any way, considering 3 shots with a Plasma Cannon take at least 9.9 seconds (with rapid reload to V) to let off. This is not accounting for the time it takes to aim, the time it takes the round to reach the target, or the short ~0.25 second lockup that happens after firing. This puts the TTK to ~10.4 seconds at point blank range, where aiming doesn't take much time at all, and the rounds make contact almost immediately.

These are the optimal TTKs for the different HAVs at point blank range, using an Allotek Plasma Cannon, with Rapid Reload to level V

Gunnlogi: ~5.05 seconds
Madrugar: ~10.4 seconds

Using an ADV or STD Plasma Cannon, with Rapid Reload V:

Gunnlogi: ~10.4 seconds
Madrugar: ~15.35 seconds

(These next numbers are speculation, and they are my rough estimates, based on experience)
At ranges of 30-50m expect those numbers to increase by about 15%
At ranges of 60-100m expect those numbers to increase by about 40%
At ranges of 100-250m expect those numbers to increase by about 120%
This assumes all the rounds hit at these ranges, and unless you are a machine, you will never land every round at extreme ranges.

In closing, I believe 2500 direct damage is reasonable.

Yes a buff splash could be significant but not a buff of the splash damage to prevent the weapon becomes op.