I have close to 10,000 potential clients and yet 75 of them are currently
using the backup service on a VM server.

I do not know, exactly what would be the capacity   (that depends, ofcourse).
With multi-tasking over mult-processors, processor may not be bottleneck. I
gusess the first bottleneck will be the 3172. With the introduction of
OS/2 or AIX servers, we can distribute the backup service over different
buildings - I think it may help if the AIX or OS/2 Servers can be configured
to talk to the VM or MVS server. For example:

Use several AIX or OS/2 servers and define their last destination to point
to VM or MVS server. This will help me. What do you think ?