I can't find the specific motherboard used by the Mini X+. It's either the Supermicro A2SDi-8C+-HLN4F, the C3758 version of the board in the Mini X, or a custom version with 10GbE instead of the quad GbE. If it's the latter case, the C3758 SoC supports 10GbE, 2.5GbE and 1GbE, PHY support allowing. Since 2.5GbE is 10GbE encoding at 1GbE bandwidth, I would assume it would be supported in all C3758 10GbE systems.
Honestly, send an email to the iX sales team and they'll probably answer very quickly. Let us know their answer.