I am quite familiar with the Aja System test and I get close to 200mb/s between the Xserve and the external raid array. Over the network i get close to 80mb/s. I have a modern Intel Xserve which has jumbo frames and a small tree network card. The only old things are the clients which are G5s but they do have jumbo frames enabled. I have L2 Managed Netgear switches. All systems are running 10.5.8. I have tried with the clients directly connected to the Xserve. Now I have them connected via the switches with both the switches and the Xserve having link aggregation enabled.
I am not a whiner or a complainer and I understand these things take time and money. I am willing to spend and have spent both.