I meant the kind of bandwidth that GDDR5X offers in a 256 bit config. Could have been 384 bit GDDR5 when in the drawing board.
I don't think 384 bits was never on the table for this class of chip.
GDDR5 at 8Gbps is already 15% faster than 7Gbps.
I use the rule of thumb that an x% increase in memory clock speed results in an x/2% increase in performance, based on the overclocking table in Anandtech's GTX980 review. And that the first order opposite is true as well: a reduction in BW only results in half the performance loss.
But this means that a lot of work segments are not memory controller limited.
A GTX 980 Ti has 336 GB/s. A GDDR5 gp104 will have 256 GB/s. That's 30% less, but only 15% less in performance. If you now significantly increase the performance of the compute bound work segments, you should be able to make up for that 15% loss. Going from 3072 cores at 1.1GHz to less than that but at much higher clocks. According to that same overclock table, core clock increases by x% increase performance by more than x/2%.
Add it some architectural improvements and there you have it: 980 Ti level performance with old school GDDR5 at lower cost, a perfect replacement for gm204. The GDDR5X is the cherry on the cake to make a true new performance leader, with no competitor in sight.
Cue in everybody complaining again how Nvidia dares to ask high-end prices for a mid-end chip.