I remember when I was really happy to bump FTP performance from about 70kbps to 300kbps on the 3Mbps ethernet (yeah, those are all lower-case "b" for bits.) It seems that much before that, 56kbps was about the max link speed anyway, so no one had really noticed how slow the internet code was 
would you be interested in a faster [ethernet]?
I dunno. I'd rather have a board that had a better overall design for networking. If it's going to use SPI, it should at least have DMA for it's SPI interfaces. (some XMEGA chips can do that, I think.) Overall speed is probably less interesting than "overhead"; I have to generate the data I'm sending somehow...
Did you investigate using the parallel interface mode of the W5100 ? That seems more natural than putting a custom SPI controller in front of the SPI interface. You could even consider using a more intensive parallel interface to some other ethernet controller (say, like a Yun with higher speed interconnect between the ARM and AVR.)
There are only so many "fixes" you can make to an AVR-based networking device before you have to ask yourself "why was I not using a Raspberry Pi or BeagleBone Black, again?" (or even: connect up the Ethernet on a Due-like design...)