Specifically will my carrera lightweights for my 2003 C2 fit a C4S if a 17mm spacer techart spacer is used. I could have sworn the factory's official use for these spacers is to fit narrow body wheels on wide body.

If the answer is yes, then it makes my sale of my winter wheels easier.

 

yes they will
i put 18" narrowbody TechArt wheels on my widebody 4S for my winterwheels. 15-17mm spacers should do the trick

 

GT3 wheels fit nicely on a C4S for winter tires/wheels as well (no spacers required).