It seems HBM is still a PITA and still won't be available in quantity until way later in the year.
GDDR5X still allows double density over GDDR5 so you could have 24GB on a 384 bit bus. 32 on a 512.
GDDR5x offers zero density improvement over gddr5. Gddr5 has been around for ages in 4Gb capacity but has been in production of 8Gb chips for some time, gddr5x is only 8Gb chips, it's just a dumb association that somehow gddr5 is 4Gb as standard because 8Gb chips haven't really made it into cards yet.
It will offer higher speeds, the key advantage really is higher speeds on a smaller bus. How much extra power is used with gddr5x at say 12Gbps vs gddr5 at 8Gbps, who knows. Theoretically you could have a 384bit bus using gddr5 8Gbps chips or 256bit bus and gddr5x 12Gbps chips and have the same bandwidth. 256bit bus would save die space, 384bit bus clocked lower might not use as much power.
Realistically if a high end 6-8TF/s chip needs 300+ Gb/s bandwidth today, a midrange gpu will need that in the next gen. Could mean midrange 350mm^2 Polaris having say a 384bit bus with gddr5x rather than needing a 512bit bus.
From what I've heard about HBM yields and interposer connecting yields and frankly Samsung/Hynix ability to produce memory I doubt the memory itself is any problem. Before Fury there was no effective production chain for attaching silicon chips to other silicon chips. We're talking a whole magnitude or two lower in scale when attaching TSV's as opposed to soldering bumps. You pretty much need foundry level clean room facilities to stick chips onto interposers. Producing chips is as easy to ramp up as any other chip before it. You have plants, you put in a different set of masks as they do today to switch between thousands of products being produced. Overnight you can turn down gddr5 production by 30% and replace it with HBM, that isn't even an issue. The issue is putting it all together on an interposer, that is almost certainly where the production bottleneck is and as with foundries, it won't be fixed overnight.
While you can switch gddr5 to hbm production literally overnight, you can't switch a current packaging facility over to interposer packaging overnight, you can't do it over a year, it's a multi year thing.
It won't take years from now, building will have started a long while back, but as with foundries you start something small and scale up.
