When you changed to metric did you then scale the drawing - or at least tick the box in the space unit dialog that says 'convert units after alteration', (I'm no too keen in the auto convert thing).
Someone will correct me if I'm wrong - but this is my understanding,
The problem is we are going through three essentially incompatible systems, TC which can be set as English or metric scales, stl which is unit-less i.e. '1' of whatever in TC = '1' in stl whether its inches, millimetres or furlongs, stl doesn't know its 'inch' from its 'mm', all it sees is 1=1, then you have cura which the programmers decided what scale it works in, somewhere along the line - we as users must work to accommodate these changes.
When TC (or any program) saves as stl it saves 1 of whatever unit we are working in, and converts it to 1 unit in stl, so 1 inch = 1 stl unit and 1 mm = 1 stl unit. its up to us to set the necessary scale, whether that is by altering the file in TC, or using the scale factor in Cura is up the the user, essentially if Cura's base units were inches, I as a metric user would have to rescale my whole drawings by 0.03937 (approximately)
Basically there are only two options, either rescale the drawing in TC (either by changing to metric and ticking convert, or changing to metric, select the whole drawing and multiplying by 25.4 (in x y and z)), or second option of noting what the units are in TC and scaling in Cura by 25.4 or whatever.
not sure if that explains it well, but its how I see it.