Soundstage, soundstage ... personally, I think that's a dicey quality to hang your hat on a judgment of differences. All of the headphones are constructed the same from SR to RS series. The only difference is in the choice of materials and attention to driver matching. And perhaps most important if we're judging "soundstage" - in the low-priced versions they use flat pads. Time was before the Apple-coat-tail "i" versions of Grados, the RS-80 on up had bowls for pads. IMHO, there is no difference in soundstage among any of the models if the ear pads are the same.
Does soundstage increase from flat pads to bowls? Yes. So in the current "i" versions, it might be accurate to state that soundstage increases from the RS-80 to the RS-125 on up. That's only a change in pads, though, nothing more that's going to effect soundstage. You do get somewhat of an increased airiness because the screens get more porous in the higher-priced versions, but that's a minor effect, IMHO. The key is all in the chambers, IMHO. The HF-1 comes very close to the RS-1 in sound, but in appearance it looks like an RS-60/80/125/225 except for wood buttons (old versions with bowls, though). The major difference in the phones is that hidden inside is a mahogany cup - not plastic.
Since the GS-1000's use the giant bowls, then yeah - I bet the soundstage changes once more. Most people who like Grado's seem a bit put off by the GS-1000's, though.