Hi,
I'll just try to take a stab at this again and hope I'm not irritating the crap out of you. While I do use Mac and I have helped a little with a University installation, I haven't done anything in the last few years. I just use NFS at home.
Basically, as you probably know, the streams method of handling files is sort of broken in the unix/windows world. If you were using streams files before, then the migration probably should have been done with a Mac instead of robocopy.
You might look at DAVE/ADmitMac and see if a test license is possible. The software does on the client side what your Z-IP software does/did. The DAVE software is sort of supported, IIRC
NFS (without the streams fork) is a pretty good option if you have an NFS license. You might be able to obtain one for testing, if not.
There is at least one Mac related bug in ONTap 8.0.1, but it doesn't look like it directly applies to you. If you have the chance, running 8.0.1P1 or 8.0.1P3 might help (as a sort of unscientific wild guess).
The last option is, of course, to try to sniff the relevant network connections. I don't know how busy your filer is, but you have alternatives on both sides when using a Mac: tcpdump on the Mac side and pktt on the filer side. Wireshark should give you a chance to examine things even as a semi-layman. If not, it is a good basis for opening a case with IBM/NetApp.
So, now I'll shut up. I wish you luck.