So this attachment upgrades the ability of stealth twice. That's great. I asked my GM if that applies to when it's perception vs stealth, he said yes at the time, but I'm not sure, and this answer is obscure enough that I haven't been able to find anything one way or the other.

To clarify if I have this attachment and an NPC rolls his skill vs my skill, as in my skill is negative dice, does the upgrade still happen?

Clearly it should work, it would make no real sense if it didn't I just wanted to make sure I wasn't cheating when I go to buy it.

It obviously would upgrade his Difficulty in the case of the NPC making an Opposed roll. Although your GM should be making you roll as much actively as possible, so you should be rolling Stealth vs his Perception.

Yes. It would upgrade the difficulty in the event it was an NPCs perception vs your stealth.

Cool, I agree that I should roll, but I can see secret rolls being made in this case. I know that in principle secret rolls shouldn't be made in this system, but sometimes they can be useful. Thanks guys.