@MarkAGregory 

The module doesnt negotiate with the switch, the SFP+ side of the module is FIXED at 10g, the switch has to match.  You can test this by plugging it into a SFP (not plus, 1g port) and nothing will happen.  The module is what negotiates the RJ45 side and does the internal speed conversion to the 10g side.  Most modules will happily do the negotiation for 1g,2.5, sometimes 5 and 10g, it depends on the module specs itself - they effectively act as a 2 port switch themselves with no further intelligence or configuration options.

you CAN change switch port speed for DAC cables that are rated for 10g, because these are capable of running at 1g as well.

  @GRL this is what I said. This does not prevent the module from connecting to a device at 2.5 GE. I connected several and after a few steps they all worked. A 10 GE port can carry traffic at 2.5 GE. I would have liked to try a couple of 5GE devices but did not have any handy.

Also, setting the port to 1 GE permits the module to support devices at 1 GE without any additional steps.