Under-clocking an 10gb Ethernet SFP+ module
A lot of the simpler SFP+ transceivers (direct attach copper cable, short-reach optical fiber) are really little more than analog devices and wouldn't care about the clock (fun fact: friends of mine use SFP optical transmitters to simply distribute a 10 MHz clock, and it works).

None of the stock networking hardware would be prepaed for a 50% rate mismatch, and I'd doubt their PLLs would lock reliably.

So, while point-to-point between devices using relatively dumb SFP+ transceivers might work with arbitrary clocks, I'd not expect functionality with stock networking hardware. It's more likely that you can convince your Zynq's transmitter to work at the full rate than that...

I don't know whether that's viable on your board, but maybe you could use the PCIe endpoints to have 2 or 4 lanes to a 10 GigE network card.