Under the old rules when boosted was so openly allowed, the Stock/Spec made the most of the classes that had a "spec" motor.(ie 10.5, 13.5, 17.5) Although, if your track has a "spec" motor class which allows boosted speed controls, the SXX V2 Stock /Spec is the way to go. Hence the name Stock / Spec with the emphasis on "Spec".
However nowadays its either "non-boost" or "mod". In "mod" its anything goes so you can run more motor with less boost or less motor with more boost. Whichever way gets you around the track the fastest and smoothest is what wins.