When using the UltraScale 10GBASE-KR core, TXOUTCLK changes frequency and should not be shared between multiple cores.
The GT is initially configuration to have the asynchronous gearbox disabled and TXOUTCLK of 322Mhz.
After training and AN complete, the core uses a DRP interface to enable the asynchronous gearbox, and TXOUTCLK is set to 312.5Mhz.
Even when AN and LT are disabled, the initial GT configuration will be set for 322Mhz TXOUTCLK out of configuration and reset.
For an UltraScale 10GBASE-KR core, each core should use its own TXOUTCLK. (PG068) will to be updated to reflect this in the 2016.1 release.
If you are using a 10GBASER or 7 Series 10GBASE-KR core, the configured TXOUTCLK frequency will not change, but care should be taken as TXOUTCLK is lost whenever the main or PMA reset is issued.