Ok that is great news 
How about the stability on that kind of module? What library do I need to get it work and what in the code is difference from to use with the Ethernet lib... if you cannot use the Ethernet?
Regarding the W5100 shield my condition are:
Connect the Arduino Mega to the internet and connect the board to a computer program called Prosim737. (Flight Simulator). The mega will then do something with the sketch and components and take commands like switch a switch and send it back to the Prosim737.
Sum:
I do something with my components and send it through my network to Prosim737 which will receive my components state and execute what I did with my components.
The question is: Is the Shield with the W5100 fast enouth to execute my commands in 1-2 sec or something like that?
Will I note the time between a w5100 and the W5500?
Best regards Fredrik