I am not so sure about consistent quality differences in terms of sound and playablility.
I have owned a total of 8 different SGs and LP standards, specials and studios over the years, all i can say are that because its all handmade, its really careful selection that gets you that prized piece.
Honestly, the "studio" and "standard" tags are really cosmetic considerations. The "Gibson" sound is always present there, I know...i still own 7 different Gibson models. Only thing is each piece has its nuances because they are all made individually by human hands and not machine pressed.
And to all the naysayers to Gibsons, its all about the set up too and playing it in. Own one first for a period of time and try it against all amps in all situations, and you will know why some of us are Gibson fanatics.