Tried Infiniband? The switches cost a fortune, but you can pick up 8Gb/s interfaces dirt cheap. Connect a cable between them and you've a point-to-point link. It does a decent bitrate, and it can do RDMA too, so if you get it configured right (which is a bit tricky) you can get some impressive performance out of it. The prices do get high if you want more than two computers on your network though.