Suggestions about to improve Net-work performance

One of our very important customers is doing some tests using Net-work connecting Mainframe (z/OS - WCP591) with AIX (WCP723) via TCP/IP.
They would like to know what are our recommendations to improve the performance because current results are not good.
They are using Natural to access Adabas files from both nodes.
They are thinking to have about 700 concurrent users executing Natural applications from Mainframe and AIX.

Do you have some recommendations/suggestions to follow to improve the performance ? about WCP parameters, TCP/IP parms, buffer sizes, bandwidth, configurations, etc.

All suggestions are welcomed.
Thanks and regards,

I am sure a lot of developers would find that information useful - if anyone has had any experience with this.