10 pen doesn't seem like much.

It's roughly 6% more damage IF the opponent has above base toughness. (Worth it on Boltgun and that's about it)

I find it handy for marines and Orks usually, certainly can help with Eldar, but I've not noticed an overwhelming change in their ability to survive with or without toughness.

At least for the bolter, it usually does not lower your Shots-To-Kill in most situations. Dunno for the Shootah.

Use it on your Boltgun. It never hurts to have some extra punch.

It makes you kill tougness enemies 1 bullet faster. It does nothing if they have base toughness

That's not true, usually it reduces shots to kill by one as long as they have any toughness boosting at all.

TBH i prefer having a drum magazine, double the amount of ammo is better than about 6% extra damage for me.

The base TTK is too low to actually make this viable in my opinion.

Exactly this! They should just introduce another mod slot, so that mag and ammo are separate! With more ammo and mag mods comming in the latest patches, there's more than enough choices.