would like to know for sure that the die is 81mm²... dies are rarely exactly 9x9 mm... so it'd be nice to know exactly how big it is to at least one decimal place rather than the very rough and unlikely sounding 9x9mm number. e.g.,if it was 9.5x9.5mm that'd be a 90mm² die.
makes a bigger difference when we're trying to calculate the gh per mm²
i also heard for instance and by comparison that knc has a 6.6mm x 6.6mm die = 43.5mm², which hashes at 650 GH for 4 chips, and each chip has 4 dies.. thus each die hashes at 40.625 GH... thus the gh/mm is 0.94 gh/mm²
Amy from HF wrote about this a couple of months back.
HashFast's Golden Nonce chip: I don't have to estimate the size because I work at HashFast. 
One 18x18mm die is able to do 400 GHash (nominal - more overclocked**)
Hashing per square mm:
18x18mm = 324mm^2
400 GHash / 324mm^2 = 1.23 GHash/mm^2
One 18x18mm die is able to do 400 GHash (nominal - more overclocked**)
Hashing per square mm:
18x18mm = 324mm^2
400 GHash / 324mm^2 = 1.23 GHash/mm^2
So I guess the 18x18 is really 4x9x9