There will be no issue running the I/O powered with VCCAUX_IO = 2V at data rates that are supported by the VCCAUX_IO 1.8V.
Increasing the VCCAUX_IO to 2V will increase power consumption (use XPE or Xpower for numbers). Also 1.8V is a more common power rail and would allow for the sharing of supplies, using 2V requires a dedicated regulator for this. If you are using LVDS VCCAUX_IO, it can be set at 1.8 or 2.0V and meet the 1.25 Gb/s.
Running at 2.0V will be better for performance, but burns more power; therefore, use VCCAUX_IO = 1.8V, if possible.