It is an axis problem. When converting, the axis of the GROUP, not the object, is what gets translated into the object center in FS. I think it can be fixed by hitting the "move axis to center of object" button in Truespace, but I'm not sure if it calculates the physical center correctly in a group. Plus you're going to have to redefine your fire points relative to the new axis coordinates. And place the light at the same location as the turret before you glue next time.
EDIT:
From Dictionary.com:
turret:
[list=1]
- A small tower or tower-shaped projection on a building.
a) A low, heavily armored structure, usually rotating horizontally, containing mounted guns and their gunners or crew, as on a warship or tank.
b) A domelike gunner's enclosure projecting from the fuselage of a combat aircraft.
[/list=1]
-and-No entry found for turrent.