Do you say "reduce something to polystyrene"?
Just write what you actually did: You used the calibration curve of your SEC column for polystyrene in solvent xx. Those phrases you cite are meant to sound impressive, but are actually just scientifically wrong, bad English, and therefore stupid. 

Nothing gets reduced here like a reduced temperature or a chemical reduction. Also there is no "PS equivalent Mw", because it depends strongly on the solvent. "Relative" is as wrong, because that implies that there is simply a factor between the two scales, while in fact there are two calibration curves, one of which is unknown.