Yes, Heroes in Clash of Clans are extremely important and I am not say that above level 15 Heroes are not important. I am talking about the first 15 levels of 40 upgrades, which give you the greatest value, especially at Town Hall 8 and 9 players, this is heavily prioritized.
First, first 15 levels are cheapest and quickest (obviously). At these level, our King and Queen gain a lot of goodies along the way, which significantly change both of their damage and abilities (for example, the main diffidence between level 31 Archer Queen vs level 30 Archer Queen is that level 30 Queen can 1 hit Builder Huts).
Why 15? Why not 10 or 20?
It’s because there are some amazing factors I want to show you in this post :D.
Obviously you want to upgrade your Heroes to level 5 as soon as possible as they gain their special abilities at level 5: The Barbarian King instantly regenerates a lot of HP, summons some Barbarians and boosts himself as well as all Barbarians nearby; The Queen becomes invisible immediately, gets better damage, attack speed and also summons some Archers.
At level 10, Hero abilities get upgraded to the next level. While using the ability, Queen’s damage gets increased by approximately 16% when compared with Royal Cloak level 1. That’s a lot! At this level, she has enough time to shot 5 times. This only happens again when she is at level 30, she could hit 6 times with the ability. Level 10 Barbarian King with the level 2 ability (available at Town Hall 8) receives a major upgrade in respect. Lvl 5 raged King deals 222 damage per hit while lvl 10 raged King deals 292 damage per hit. The extra 70 damage might not seem like much but the level 2 ability is almost 32% stronger than the level 1 ability (meanwhile level 8 ability, the maxed one, is about 14% stronger than the level 7 ability). Sounds awesome?
Likewise, level 3 Iron Fist is approximately 25% stronger than level 2 Iron Fist. This is not good as above but it is still a massive increase. At this point, most players, who are reading this, probably think that the level 4 Iron First has a smaller boost than Iron First level 3 and so on, why do I choose the line at level 3 ability? It’s because of the Rage Spell! Basically, a maximum Rage Spell allows our King (at level 15 and below) to do a lot of more damage than when he is using his ability.
The level 5 P.E.K.K.A has the highest damage per hit in game, she can 1 hit level 14 Queen but can’t 1 hit level 15 Queen. This is pretty important, especially when you use Super Queen strategy.
Finally, below level 15, it takes you less than 1 week to upgrade your Heroes.
At Town Hall 7, upgrading your King to level 5 is the best use of your Dark Elixir. Using Hog Rider strategy at Town Hall 7 is not a good idea at all. I strongly recommend using only Dragons for Clan Wars.
At Town Hall 8, although the boost of the level 2 Iron Fist is awesome, I still recommend upgrading your Hogs to level 3 first then you can upgrade your Hogs to level 4 while working on the Barbarian King. Level 4 Hogs are extremely useful in Clan Wars.
At Town Hall 9, just get the Queen mediately and get her to level 5 regardless of your King level. You can start using Dark Elixir on your troops after getting level 5 Queen although I would like to recommend you get her to level 10. If your King is not at level 10 at this time, you should also upgrade him because of the significantly strong level 2 ability. Get him to level 10 before the Queen!