Well, the Gun skill also adds a small but steady increase to your gun damage as your skill increases, doesn't it? So there wouldn't really be a "max" where it stops being useful, and it would be one of the few skills that's worth perpetually keeping as high as possible. Or am I incorrect in this assumption?
If this is true, then Wildan is correct indeed and every extra point should be put into Perception and four extra points in perception are worth more than gun nut feat.
At level 25 you can max your base Guns skill at 135, but with 16 Perception the effective Guns skill is 272! Compared to 203 with Perception at level 10 or 180 at level 8.
Wiki says: "The formula for damage gained from Guns skill is Base Damage * (1 + 0.7 * Effective Guns Skill / 100)."
Lets do some maths. If you want to have Co:10 and Int:7 (for gun nut feat) you can raise your perception to 12. This gives maximum Effective guns skill 226
Damage multiplier = 2,582
Effective guns skill 272
Damage multiplier = 2,904
Let's take a weapon, which has a basic damage of 20.
With Gun Nut you can make it 24 and with Guns 226 multiplier added the damage is 61,968
Without Gun Nut, with Guns at 272. The damage is 20*2,904=58,08
Slightly less, but very slightly, since the accuracy bonus from Guns skill make difference at low levels.
So, if this holds true, I guess Wildan is right and all the extra points should indeed be put into Perception.