You're uninformed then. The reason for adding 6+GB is to add more bandwidth while the ram largely isn't fully utilized. Check the current benchmarks without optimized drivers. It has the same or better performance than 980Ti and sometimes Titan X. Once directx 12 and openCL 2.x are utilized, because it uses IP developed by AMD, AMD cards will pull away significantly over Nvidia, even with, as you say, only 4GB memory. Now maybe the 8GB fury X variant greatly outperforms, I don't know. But if you truly look at the numbers, the three month driver optimization curve AMD usually has for greatly improving performance, and the above new APIs, it only makes sense to buy AMD!!!
Edit: To speak to your "rush job" comment, AMD was going to release info on the card back in March until the Titan X was released right before they were going to talk, that way to steal AMDs thunder. Then, they announced E3 for the event in May. There was nothing rushed about it. Further, fears from Nvidia caused it to sacrifice Titan X sales because they thought performance of Fury X would be great enough to drown both Titan X and 980Ti sales. It forced a reduction of $200 on the rumored release price to match the 980Ti, potentially costing millions in margin on the card. It wasn't rushed and this is Nvidia fanboy rhetoric!!!