The 780 Ti was at 533 mm^2 vs the 290X's 455 mm^2.
Slight correction:
780Ti = 561mm2
290X = 438mm2
AMD moving from 438mm2 to 550mm2 is a 25.5% increase in die size.
NV moving from 561mm2 to say 620mm2 is an 10.5% increase in die size.
AMD actually quoted a 352mm2 die size for Tahiti XT despite GPU-Z reporting about 365mm2.
Going from 352mm2 7970Ghz to a 438mm2 R9 290X is a 35% increase in performance with a 24.4% increase in die size. If R9 380X is 550mm2, it should automatically have 35% more perfomrance over the 290X, if AMD manages to reduce power usage.
Anything extra will come from the improvements in improvements in efficiency (for example more tightly grouped GCN cores, larger L2 cache, more efficient TMUs, ROPs and SPs), and memory bandwidth and latency reduction efficiencies (HBM). I would say 45-50% over the R9 290X isn't out out f the question at 300W power usage. Anything beyond that sounds like some magical AMD engineering voodoo on 28nm. :awe: