SFP+ works, I tested with my XGS1250-12 (which has SFP+ as well). Ethtool shows 10 Gbps link speed. Hotplugging it doesn't work, but when the transceiver's plugged in at boot time, the link will come up. I used a 2m Digitus DAC.
See post #492 as well.
Edit: @robimarko @adron I initially overlooked post #492, but it looks like @sumo is seeing the same issue with the QCA 2,5G port: no link on 2,5G, but functions when the other end can only link up at 1 Gbps. Only difference to me seems to be that the port now advertises the 2500 Mbps link, whereas in sumo's port it didn't.