performance statistics with gigE on NT

I have a customer who is running a fast ethernet network. They cannot get the throughput to run 4 tape drives simultaneously to get 24mb a sec. of throughput. we have suggested to them that they go to gige cards and they should get performance numbers in the 70 mb range. They are asking us to prove this to them before they order the software and library. I have looked all over and can’t find a white paper that gives statistics of throughput with GigE cards in an NT environment. Does anybody know where a paper exists. I would appreciate any feedback.