I just got a print order for a 24"x32"...I am having second thoughts, and may use Epson Luster...this has been my standard paper, and I know I can flatten it.
You may want to consider Canson Baryta Photographique. I haven't had any problems decurling from rolls (9890). It is almost identical to GFS, except the base paper. I suspect the top baryta layer is applied by the same mill.