Can you check you MSS value after the connection is established?
I have wondered if writing to the SnMSSR register makes a difference.
Something must be up with your MSS. I have used several makes and models of routers, and none had a MTU over 1500. If the w5100 tried a 1515 byte packet through any of my routers, they would all return that message. The packet is too big for me. Break it up into smaller pieces.
So...1460 (your MSS) + 54 (your header size) = 1514 (too big)
But 10/100 routers are still limited to 1500 bytes (1492 for the original ethernet spec. Mine are ethernet V2).
I have no idea how they picked 1492.
Maybe there is some historical significance to that number.
But there is another reason for this post. After some experimenting this morning, I found an interesting phenomenon.