As we know, CLKper4 can be appeared on some port pins (except A series) and the maximum allowable frequency for this clock is 128MHz. Also according to datasheets, the typical pin rise time is 4ns. I asked Atmel support for maximum frequency of port pins (PC7,PD7,...) when CLKper4 will be output to port pin, but the answer does not contain any new information:
For XMEGA AU devices, you can choose which of the peripheral clocks
(CLKper, CLKper2 or CLKper4) will be output to the port pin with the
CLKOUTSEL bits in the CLKEVOUT register.
The clock output option on XMEGA A devices can only output the CLKper.