If I understand the formula for calculating damage from an attack correctly, the graph I am attaching shows how the factor related to level works. A lvl 30 attacking a lvl 1 does almost 8 times the damage it would against another lvl 30. Conversely, a lvl 30 attacking a lvl 40 does more than 90% of the damage it would do to another lvl 30. Likewise, a lvl 1 attacking a lvl 30 barely tickles, but a lvl 40 attacking a lvl 30 does around 10% more damage than another lvl 30 attacking.
It could be that small increments, over the course of a battle, add up to a significant difference, or there are other advantages to leveling up (more hp, scarier looking number in the gym, etc.), but gosh, its a lot of candy and dust to get those extra little increments.
Good analysis. I think it matters less for attackers because with dodging, a 2000 CP Vaporeon can take out a 3000 CP Snorlax and then some.
For defenders, it's all about gym placement, so more CP = better placement.
Personally, I power up my favorite mons to their highest levels because I enjoy it. I can't really tell you how much of a difference it actually makes in practice. I typically start with defenders though and only power up attackers if I have candy and stardust to spare.
Yes, it is worth it but only for your attacking and defending A team.
I mostly max out my defenders which consists of an army of Snorlaxs, Lapris and Vaporeons. And my Attackers which are Dragonites and Gyarados. Dust is easily obtainable by hatching eggs and collecting a 10 gym defender every 21 hours on the dot.
In actual practice, over LVl 30 powerups mean 1-2 seconds faster at most, not significant...FuNKyShO does a lot of comparison videos and shows actual gameplay of this here below (Lvl 35.5 vs 30):
https://www.youtube.com/watch?v=8PDsZuUjEjk
For defenders, it is absolutely worth it as a lvl 30 vs lvl 40 CP would be about 400 CP (20 powerups?) difference which means top of the gym or bottom/not even in the gym placement.
For common mons which you can get an endless supply of candy (any Eevee evolution), it's easily possible and worth it early to keep them all maxed if stardust is not a problem (less at Lvl 34+ due to slow leveling).
For rares like if you have multiple Snorlax, you may want to prioritize your defenders first.
