Granted, an B13 NX coupe or N14 is a bolt in installation, which offer the next lightest body shells.
Of these, the NX offers better aerodynamics.
I'd guess there is about a 100kg penality in the weight of the body shell, given the weights below.
That would then be penalties of 160kg for S13 Silvia, 200kg for S13 180SX or GTiR, & these all have less efficient drive trains (i.e. power turned through 90 degrees).
K11 2dr CG10DE 720kg (incl power steer)
K11 2dr CG13DE 810kg
K12 2dr CR12DE 870kg
B13 NX GA15DS 940kg
N14 2dr hatch GA15DS 940kg
B13 NX SR18DE 1050kg
N14 2dr hatch SR18DE 1050kg
N15 2dr hatch SR18DE 1120kg
S13 silvia SR20DE 1110kg
S14 silvia SR20DE 1140kg
S14 silvia facelift SR20DE 1160kg
S15 silvia SR20DE 1200kg
N14 GTiR SR20DET 1190kg
S13 silvia SR20DET 1150kg
S13 180SX SR20DET 1190kg
S14 silvia SR20DET 1220kg
S14 silvia facelift SR20DET 1240kg
S15 silvia SR20DET 1240kg
All weights are for manual gearboxes & the lesser specification available for each engine size (i.e. no sunroofs, HICAS etc)