Yep its not a practical tower just to see what I can get for max dmg :P. The other one isnt the best setup either imo because i think its better to have an area burst tower mixed in one of the mega mega boost towers to clean up all the extra nonghosts, rather than just haveing pure boostage in the other towers. You can still get ~21 million (maybe higher actually using the storage method) in the main dmg tower and have a 3 million+ area burst tower.
Just made this tower as someone was mentioning with the bug around if you have damage boost towers around at the time of combining towers, the damage being increased.
Basically, the way I did this was to make as many damage boost towers as possible around the top-left Mega Tower making the MMT, then deleted the boost towers at the top right MT of the MMT, etc etc. Basically, taking full advantage of that little bug. I suspect that this could be the maximum damage available with splash towers only, but don't quote me on that!
Adjusting that last attempt by adding in range and reload towers as appropriate, i get 26.8mil, which is probably as good as it gets... Unless damage+range MTs get more than pure damage MTs? Dunno on that...
It made from 9 splash tower, 1 sniper, the rest is dmg boost. It wasnt too easy to create it. The idea was that constantly reselling everything, except the main tower. It took me around ~1h to create it :P