I shall explain this... once again.
The Damage upgrades have a total of three functions:
- Increase % damage
- Increase splash radius
- Increase splash damage.
Both the first two work properly. If you are hit by the beam of an upgraded tower, you take 200 damage, mitigated by armor, and everything around you by 2 feet takes splash damage (this was changed... originally there was no splash damage but it was added for balance reasons, which is why the second half is broken...).
If you are hit by the beat of an upgraded tower, you take 220 damage, mitigated by armor, and everything around you by 2.5 feet takes splash damage. That part works.
However, unupgraded you take 100% splash damage. That is, 200.
Upgraded, you take 10/20/30/40/50% splash damage. That's 22, 25... and so on.
So... if a demigod is shot by the tower, having it upgraded is more useful. However, an upgraded tower takes longer to kill minions and creep waves. Which is why buying it is bad.