I don't know.
But here's a simple test.
Go to your AIX box, find a file that is at least 100 MB, preferably 500MB.

Send it to your OS/390 mainframe via a simple FTP, and time it.
Do it 3 or 4 times to make sure you are getting consistent timings.

FTP is about the simplest data transfer you can do via TCP/IP.
TSM won't ever go any faster than FTP.

That will take TSM issues out of the picture, and see if you can push a
simple data transfer any faster to your OS/390 machine.

(BTW, you didn't mention your release of OS/390.  I think at about OS/390
R4, there were significant changes that improved the performance of OS/390
TCP/IP.)