See this thread for the ongoing (as of 10/12/2015) discussion with the Dev j.harshaw responsible for balancing.
ATM (pre-release of v4, ie beta) :
- A Low-grade mod will adjust 1 stat of your weapon (cooldown time, reload time, damage, number of 'particles' (ie ricochets) for shotgun etc)
- A Medium-grade mod will adjust 2 stats
- A High-grade mod will adjust 4 stats
Stats may go down as well as up!
Only station mechanics can fit (or remove) a mod, but you can dismantle a mod at any time to recraft it into something with a different set of stats.
Mod crafting reqires mod parts & Xenon components. The result will be a random mod of the same grade with random stats