I am not an expert using SFP.
I got a Cisco SG500X L3 switch with 4 SFP+ 10G ports.
I got 2 mini-gbic HP AJ718A (8Gbps) and 1 fiber (HP 3PAR-10G-10M 850-200029 10Gbps). I plug the fiber into XG1 & XG2 ports (I know it's a loop but STP is enabled), and I got 2 ports at 1000Mbps according to the webconsole.
I got 1 all-in-one cable 10G (10Gtek SFP-DAC) and in the same ports i got 10Gbps connected ports.
I don't understand why. The switch is unable to works at 8Gbps ? It's 1 ou 10 Gbps ? Thank you