If there's a GP102 it likely means that GP100 dies binned as defective will literally be (rubbish) binned. GP102 with similar FP32 to FP64 unit ratio as the GP104 would probably be about right re: 4000+ (they say ~4500) shader units. That doesn't look wrong to me.
I'd assumed that they would have to reclaim the GP100s in consumer products, even if the performance for gaming was a total disaster. But maybe it's just too bad.
This would probably be the biggest advantage AMD have, regardless of whether they turn out to have extraordinary product performance ... the ability to use the same dies for pro graphics and compute products as gaming / multimedia. GCN units are completely agnostic; there is no single precision or double precision hardware. The components of NVIDIA's shader units are dedicated to FP32 or FP64. If they need more FP64 for double precision compute (like supercomputer type workloads), per the GP100, that cripples pro-graphics, gaming and single precision.