I was able to get my Omnia to talk to a MikroTik CRS305 at 2.5Gbps via an SFP DAC, in this forum post.

Short version: You need to force 2.5Gbps on both sides (disable auto-negotiation). Doing so requires the other side to have a switch/NIC ASIC that can talk to the SFP+ module at 2.5Gbps (this is not always supported – even if you were to run 2.5GBASE-T off a transceiver, the ASIC and transceiver may still be communicating at a higher rate). In other words, this isn’t going to work on all hardware. For the long version, read on.