I had a brainstorm, and I wanted to run it by you wonderful people and get your take on it:
This would be up to GM discretion as to which mods qualify, but it would generally be Skill and Innate Talent mods. Important to note that this doesn't extend to the base modifiers of an attachment.
Instead of a Mechanics check and credits, the character can spend 5*X (where X is the total number of mods installed after installing this mod) XP to have the mod "installed" in lieu of credits and a Mechanics check. For balance reasons (with narrative justification), this would still count towards mods for credits and Mechanics check.
First mod would be 5 XP, second 10, etc.
Now the narrative justification:
It takes some experience to know best how to take advantage of the modifications you've made, so with practice you are able to unlock the attachment's potential. But with further work, you must be careful not to disturb the fine balance you have instituted or change something that you have gotten used to. This makes it much more difficult and potentially more expensive, as you have to look for particular parts and more finely produced components, whether cutting down on weight or figuring out how to properly balance or attune a piece of technology. If you've already modified the weapon without this technique in mind, now you've got to learn that technique with the added difficulty of the stuff you changed that could throw off balance or change weight in an undesirable way.
Due to this "practice" the benefit doesn't apply to any other characters unless they also spend the XP (or in the case of an NPC, narratively "practice").
I doubt this is something I'd actually use, but I thought it was interesting and thought some of you might find it interesting. I think this is an interesting way of personalizing a weapon and truly being an "expert" with it.