The Laser Turret appears to have been a large XL2 Lasergun mounted on a turret. How it would behave is unknown, but it would most likely behave like the regular XL2 Lasergun does; it would fire a steam of lasers by quickly firing a laser from each barrel.
No stats nor model exists for the Laser Turret are in the final game's files. However, a texture for the Laser Turret can be found in the files. There is code two or so green lasers in the game's files that might've been for the Laser Turret, but since the code for the Turret no longer exists, it is impossible to determine which one would've been used for it.
Why the Laser Turret was removed is unknown. The Laser Turret appears in the manual, showing that the turret was cut very late into the game's development.
During the opening cut-scene in Sirius Downtown, Sam's ship is shot down by a slightly modified Laser Turret. However, it is only seen during the scene, and can't be found anywhere else in the level.
The Laser Turret was removed from the final game as a functional vehicle, but it's body texture is still in the game's files and it is seen in Downtown's cut-scene. This body texture is used as a part of the Plasma Turret.