Hi Marcus,
Altera only supports a lwIP HAL based ethernet driver. This means that all the packets send by software will go through the lwIP stack, and because the lwIP stack (software in general) is to slow to support 100Mbit on a Nios processor, you'll never reach a 'theoretical' 1Gbit throughput.
The problem is that software is to slow to build ethernet packets at high speed.
The only solution to this problem will probably be building the IP packets yourself and use as many hardware features as you can for high preformance. I don't know you can buy these hw features, otherwise you'll need to implement them yourself. I don't know Altera supports hw gigabit ethernet.
I need to do the same thing and this is my conclusion after reading and testing a lot.
Regards
Ronald