I doubt that there's anything specific about HBM that requires it. More likely, what we're seeing with Fiji is a result of its highly unbalanced design.
From
TechReport:
Fiji really should have more ROPs. But they ran out of room on the die and couldn't fit it. That isn't just speculation, Raja Koduri said so:
Hawaii is effective right off the bat without game-specific optimizations because it is one of AMD's most well-balanced chips.
Tahiti and Tonga also suffer from having too few ROPs per shader (in fact, the ratio is the same as in Fiji). I don't understand why they didn't fix this in Tonga. They could have cut out the useless extra 64 bits of memory controller that was never used on any shipping configuration, and used the space to enhance the ROP count from 32 to 48. (Or is there something about GCN that requires them to be added in powers of 2? Nvidia doesn't have that limitation, since GM200 has 96 ROPs.) That would have made a big difference.
Once we get 14nm Polaris GPUs, I don't think we'll see these oddball problems because they'll have plenty of room to fit a decent front end.