There were fewer R1150GSA models built (around 17,800) compared to the total R80GS numbers (about 21,800).
Will the 1150GSA be appreciated the way that some 2-Valve GS models are at some future remove?
Probably.
However, just like the earlier bikes, provenance will be important because short of getting hold of the BMW frame number lists, you can't easily tell an 1150GS with GSA bits added from a factory Adventure model. A lot of people mistake the "GSA" listing on the fiche records as denoting the Adventure, it actually means GS ABS equipped.
I'm inclined to think that a good bike will command a good price irrespective of model. The Adventure will probably just maintain the price differential over the GS in the same way that the 100GSPD fetches a bit better money than a similar condition and age 100GS (providing that the provenance shows it was a PD from the off).
Should the GSA fetch higher prices when it's really only a GS with some factory add-ons (same as a PD is just a 100GS with factory extras)? Probably not but the lower production numbers will skew prices in their favour (and probably mean quite a few 1150GSs are sold as Adventures too).