iv also seen ppl just grab a USB 3 pci card and run an USB hub thro the DVD drive door's.
there was a guy that did it but it did not look like a simple job
I think there is not much reason to do that. Upgrade the original front USB port is because of the USB ports have to be at an easy accessible location. But for LAN port, no matter if you mod the original one or add a card, it has to be at the back of the cMP anyway. Also, you must install an extra PCIe card to do that. So, you can't save a slot, you can't make the port at a more accessible location. Just greatly increase the chance to break something.
It make more sense to just instal a 10Gb card, use it, and keep the native port as a backup solution if anything goes wrong.
I bought a couple of USB3 extension cables (M->F). From the back of the MP, one comes to my desktop, and the other runs through the top handles. No terribly elegant, but not awful, either. And easy.