After some extensive testing in the gunsmith, these are the strongest attachments to equip onto the X13 Auto in Modern Warfare 3 multiplayer:
Muzzle: STER45 Skyfury Compensator
Barrel: XRK Sidewinder-6 Slide
Ammunition: 9mm High Grain Rounds
Magazine: 24 Round Mag
Rear Grip: FTAC OL-Z Grip
This combination of attachments provides a slight improvement in damage output while boosting the pistol's range and recoil control to make it a viable option in mid-range engagements as well as in close-quarters fights.
The only drawback is the visual recoil which is hugely frustrating when firing the X13 Auto for a prolonged period of time.
