in SPI.cpp, how did you get 3? And why is SPI_PCS(3) bitwise Or'd with the data to be sent? I couldn't work out what SPI_PCS(3) means.
The SAM3 SPI controller can drive multiple chip-select (SS) pins as part of the SPI transactions, and the Due SPI library allows 4 separate devices with separate configuration, one one "spi port." PCS is "peripheral Chip Select"
"3" is the default, backward-compatible with the AVR single-SS (I could have used BOARD_PIN_TO_SPI_CHANNEL(BOARD_SPI_DEFAULT_SS), from variant.h, which is what I did manually.)
The write to the SPI data register has data in the low bits and chipselect info in the high bits (apparently the upper bits get copied directly to pins, so they're active-low, which was what was causing my problems. the SPI_PCS macro fixes that.)
myspi is just a pointer to the register set having to do with that SPI port. By copying it from the library object, I avoided having to figure out whether there was more than one, or which one was used, and I could be sure that I could mix the library code with the bare metal code...
Not much speed difference than the library
To be honest, I don't know if we'll get any actual speedup at the UDP level. 8 bits every 1750ns (from your first message) is almost 5Mbps, which isn't bad (although the WizNet site says it should do 15Mbps.) At the moment this is a puzzle in SPI behavior (for me.)