XGMII is basically source synchronous,
(don't know how the pacing will be done).
To me this means an inexpensive extender/repeater
chip can easily be built.
If we use an HSTL, or impedance controlled
driver for XGMII, it should be possible to extend
the bus with several hops, and even through connectors.
At these baud rates (312.5M) it might even be possible to
retime the data without PLLs.
My conclusion is that physical distance on XGMII should not
be a big issue.