Marvin
The maximum bandwidth that you can get from a PCI Gen 3x8 slot is around 52~53 Gbps. Even though the theoretical bandwidth 64Gbps, there is some overhead on the bit encoding, CRC etc.
When using iperf, you can use -P option to create multiple parallel sessions. Depending on other settings, between 16 and 24 threads, I have been able to hit a maximum of 38+ Gbps on a 40GbE link.
If you need any assistance, please contact Support if this is post-sale. For pre-sale, please reach out to your friendly regional Sales Engineer.
Cheers
Anand