Ion canon upgrades 20-28

In my opinion the Ion canon upgrade sweet spots are at the cool down level changes,so that's how I'll break it down:

Cool down 10 turns:
Lvl --Cost--Damage
1---15----25
2---25----30
3---45----40

Cool down 9 turns:
Lvl--Cost--Damage
4---95---40
5---155---50
6---225---60
7---305---70

Cool down 8 turns:
Lvl--Cost--Damage
8---425---70
9---555---85
10--695---100
11--845---115

Cool down of 7 turns:
Lvl--Cost--Damage
12---1065--115

We can see that there is no damage increase on some upgrades, that's because like the rest of the technologies only one kind of increase is allowed per upgrade,and in this case it's either a damage increase or a cooldown decrease but not both.

I only charter the "reasonable" upgrades that are within reach of many players.So depending or the medal budget,stoping at either level 4 or level 8 or level 12 makes more sense imo.

Not upgrading the canon at all is also an option since it really is not necessary to finish the game.