As support of Daixiwen's suggestion, we have a working design that streams UDP data directly to the TSE, in 1000Base-T mode, while a Nios handles all our higher level communication (ARP, ICMP, TCP, etc.). One particularly challenging part was sharing the Avalon-ST of the TSE between the Nios and our hardware UDP stream. One needs to understand the ST protocol very well to get this to work reliably; SignalTap is your friend for this. You'll certainly need a state machine of sorts to build up your IP and UDP headers from your data stream, so a solid understanding of these protocols is important too. We burst 800MBits/s on this UDP channel, so it can be done.