I like this idea. Gun Nut is definitely one of if not the worst crafting feat right now. The damage increase is a pretty generic bonus, it's rather low and its only the upper range, making it unreliable and much less attractive to some guns, like sniper rifles. Putting more points into PER instead of INT gives much better results and saves a feat. Having both max PER and 7 INT just for an underwhelming and unreliable damage increase is hardly worth it.
A bonus mod slot on the other hand is much more creative and allows for various different bonuses in different playstyles. I believe one more slot is still not enough to install all useful mods and rob the player of meaningful choices since guns have a lot of good mods available. Quite the opposite, it would give the inferior and rarely/never used mods an opportunity to shine. Even sniper rifles, which have one more mod slot to begin with and fewer mods to choose from wouldn't suffer from lack of options. As an example, end game sniper rifle is pretty much always a spearhead with an Anatomically-Aware Scope, Rapid Reloader and Smart Module. The only mods left are Barrel Retractor and Extended Magazine, both not even worth considering in comparsion to Rapid Reloader and Smart Module. With one more slot, I could easily see myself choosing between a more close range-friendly rifle or saving some APs in big fights with fewer reloads.
At the same time I don't think it would be overpowered since most powerful mod combos tend to be mutually-exclusive as is, like, say, muzzle brake and barrel compensator.
