PCIe 3.0 runs at 8000 MegaTransfers/second for one lane.
Due to the 128/130b encoding, that means one lane can transfer 7876.9 Mbit per second after removing the overhead.
So converting to bytes, one PCIe Gen3 lane can transfer ~984.6 MB/sec.
Multiply by sixteen lanes and you get a total bandwidth of 15,754 MB/sec, or a bit shy of 16 GB/sec.