Kroom I second a 1-width Ion Beam. Give it 50% the damage and have it use 60% of the power of the vanilla part and I think it would be balanced. This would make a terrific axial weapon for fighters.
Any firing arc would be awesome, but if you add that, I would recommend lowering the part damage to roughly 30-40% of the vanilla Ion Beam to represent further reduction in beam-emitting hardware in favor of tracking hardware. This would allow for tracking by fighters on fly-by's and the construction of diagonal-axis fighter craft.
Additionally, if you want to really impress people, throwing in a reduced-damage full size Ion Beam turret with a limited firing arc would be a game-changer. This part, assuming it can turn up to 45 degrees, would allow for diagonal beam frigates.
