Awesome build, I just tested it against Archdruid Rehstin with a 10 M, 5 C, 19 D, 16 P, 10 I, 18 R gunslinger rogue. Missile Barrage did 169 dmg (deathblows, deep wounds) while he had 7.0 DR and my test rogue -6.0 with penetrating shot. One shot with lead spitter finished him.
My next MC is definitely a magic-enhanced gunslinger rogue. I really like the aspiring apprentice rp.