Okay, so we have an invasion-only attack.

We have +NPC attack/def, +base attack/def, so why no +planet attack? (not invasion)

The bonus itself, like NPC and base bonus, only apply on attacking a planet, not applied on invasion.

It can be applied on modules :

- Bombardment Cannon : +200 attack, +5% planet attack
gained via mission : Planetary Conquest Strategy

It can also be gained as a buff.

The problem is that for this weapon to have proper effectiveness it would require something like a 50% boost to planetary attack.

The reason being is thus;

With 45k attack, i can kill 2-3 population on a planet with 75k defense.

With 60k attack, i can kill 4-5 population on that same planet.

So the extra 33% attack i gained(which was 15k attack), granted me just 1-2 more dmg per shot, which is negligible in the grand schemes of depopulation.

There are already two ways to increase planet depopulation in this game, one is alerting the planet and getting your legion to smash the population with you, and the other is the Quantum Bio-Vaporizer artifact, which negates 50% of the remaining population.

maybe an upgradable module?

Because it only grants bonus for planetary attack, so maybe it is reasonable for the maximum 100% ?

What if we increase the Population and the Population damage cap? Multiplying both by 100 for example.. Would be interesting to see, and might make a bigger difference of a smaller increase in attack.

So you want to bring in a new way of how population works, just to bring something in that isn't required.

The way population works at the moment, is fine. Large population planets require the effort of your legion to deplete.

People make planets with larger population to give them more time to see the alerts and defend in time(if they're online ofc), there's already a huge way around large population planets, Quantum Bio-Vaporizers, we really don't need a cheaper way around planet population.

True, we don't really need it, i just always thought the numbers on population were way to low as it is..
I mean, did you ever see a planet in a movie or T.V show with 50,000 Pop and think that's a crazy high number? Realistically speaking, if the human race were at the level of technology of this game, and we were able to conquer and populate other planets as easily as it is done here, having only 50,000 people on one, would be insanely low, probably bordering on extinct, and in this game 50,000 population is almost unachievably high.
Just seems a bit unrealistic to me, and i have noticed Dans attention for things that, at least in theory, are realistically possible.

This is logic fallacy at its finest.
“just 1-2 more dmg per shot" is irrelevant. If the base dmg is, for example, 1-1 before buffing and 2-3 after, it's still "just 1-2 more dmg per shot" but it would have been huge, effectively more than double the dmg output and take half as many shots and thus energy.
On a planet with 1000 population, a 2.5 average will take 400 shots while a 4.5 average only 223 shots. You'd save 177 clicks and 885 energy. I daresay that much energy is fairly significant. (granted the average is NOT exact in the middle, but you got the idea)
How much extra raw value is irrelevant when considering if something is useful, only the % value and base expenditure(of energy) matters. The only reason planet attack is of lower value, compared to base and NPC, is because you usually won't spend too much base energy on a planet because there is a rather finite amount of population. (and where the population is really high, i.e. a walled planet, the defense is usually high enough such that you would be doing 1-1 after buff anyway)
But I do buff up for hyperports, which would take about same amount of energy to lock as it does to de-pop some 2k pop 20k def or so planet. So as long as the planet atk buff isn't excessively expensive, I'd use the planetary atk buff. for comparison, I'd use the 3 standard crimson buff but NOT crux for the port

Fixed.

So when this mission comes out, and it costs you 15k energy to complete, will it be worth it for the 10 seconds you'll get any use out of it, when the effect of even a 100% boost will have zero impact on your invasion chances.

Also, you complain of logic fallacy, when your own posts smacks of the same. Your own basis for belittling my post, is based on the idea that a single attacker would be taking out the population on a planet, if you're in a legion with any kind of ability to coordinate, you will have at least 10 people helping you at any time to take out a planets populous. So now your 855 energy saving is actually around a 85 energy saving per person, which is a mere pittance.

You are confusing pure logic and example. I say your logic is faulty not because of your example but because of the fact you say 1-2 dmg is insignificant, I could have pointed out that it can be 3-5 if the planet def was lower, but that's irrelevant to the LOGIC issue altogether. At the same time, if you said "saving 25% energy is insignificant when we only use a few hundred at most per person per planet", then it would be a true statement, thou possibly subjective in term of whether the difference in energy matters.
Example is always simplified. For all intend and purposes, you and your 9 pals could be working together to invade multiple planet at around the same time(being back up to each other and help each other de-pop the targets) And as such the total planet population between the planets can be 10k so it's 1k/person. There are certainly many possible scenarios, but it comes down to the same result, % matters and not the raw difference in attack dmg per shot. That is the key logic component, the actual result of the boost is % based and not raw value.
In case you didn't notice, I have said that the item is less useful than NPC/base items because the base amount tend to be lower. It can be, often times, lower than just a few hundred, but it can also be thousands per person even if you get 10-20 guys. The key factor is base value, if base value gets bigger the value of the item gets better.

I've also never said the item is very useful, looking at the module the OP proposed, if it cost 15k energy to get at its current stat, I won't go for it. It takes ~300k energy to pay itself off which I'd expect to take ~500 personal planet invasions to happen (note that while other ppl will help me with those 1k invasions, I will be also helping others during this time, so overall I'd be depopping that many planets. And I did take into account of occupying unowned ones or purchase etc, 500/planet is kind of an average I've seen across all sources, the excess being planets that everybody failed on) 500 planets is just too many. But if it was properly balanced by increasing the bonus or reducing the cost so it's down to 50-100 planets, then I just might get it. waste energy now to save energy later is fine as long as this "later" doesn't take too long. As it were, if it's at 100% boost then I might grab it.