I have an alternative Wiznet W5200 library that I've ported from work I did with on previous Arduino boards, both 328 and 1284 based. Its at https://bitbucket.org/thiseldo/ethernet52due.
This doesnt replace the w5100.cpp and w5100.h files in the main distribution Ethernet library so will be available after fitire IDE updates. I've also updated it to use the extended SPI features as well as increasing the SPI clock to 42MHz. At present the examples are configured for this library, the only change you'd need to do to use it instead of the standard library is to use the header file Ethernet52Due.h instead of Ethernet.h. The library may still have problems as its still being worked on to improve performance and reliability.
Hope this helps.
Andy