The level 1 module is the same item as the level 5 module. If you buy it when you have the level 1 skill unlocked, then you unlock to level 2, the module will change its name and stats. You don't need to buy the module again. Not the most intuitive system, I know.
Artemis reduces the amount of time it takes to get a missile lock and improves LRM tracking when you can see the target. With SRMs, it tightens the spread. The cooldown module reduces the amount of time it takes the weapon to be ready to fire again after firing. So they do different things, but yes, you can use both at the same time and they will both work.
Artemis is worthwhile for some builds but not others.
![;)](https://static.mwomercs.com/forums/public/style_emoticons/default/wink.png)
With streak missiles it improves lockon time with no drawback (the launchers do not weigh any more or take up any extra slots with artemis). That the streak missiles get a bonus is actually a bug, but it is unlikely to ever be fixed.
As for weapon modules, they also provide a bonus with no drawback, so of course they are worth using. Are they worth the cost? No, not for you, not yet. You will get more use out of the mech modules Seismic Sensor and Radar Deprivation. They are excellent with every mech.
Edited by Felio, 21 August 2015 - 03:14 PM.