Just another 5 cents...
If say, todays 6 gpu rigs push out 250-350MHs, then with VEGA's this limit is higher and already climbs to 400MHs if not more, then comes March of 2018 and NVIDIA rolls out their 20xx GPU with capability of pushing 100MHs from one PGU, having 6 GPU rig pushing 600MHs is dam close to rumorish 650MHs, the only difference would be power this rig consumes vs the power this ASIC will consume... IT WILL ALL COME TO _ROI_ at the end.
That won't work.
25MH/s require 200GB/s. 100MH/s requires 800GB/s.
You need 4 HBM2 memory dies running at 800MHz to do this. The only cards capable of being in the ballpark are the Volta based Tesla GV100 and the Titan V costing $3000. HBM2 memory has been cited as the reason for the pricing being so high. You won't get mainstream cards getting that much anytime soon.
350MH/s from a 6 GPU rig requires close to 60MH/s. Vega 64 gets ~40MH/s. RX 470/480/570/580 cards get ~30MHz, if you crank them to the max. Again you need Titan V costing $3000, or GV100 at $10K+.