Network file I/O on Windows is slow. It has little to do with how application programs such as ArcMap are written - it is Windows.
Your IT people are barking up the wrong tree. It may well be the case that a too small packet size can cause a bottleneck, but ArcGIS has nothing to do with that. ArcGIS is just using the file system, making calls to the Win32 file I/O routines. ArcGIS has no control whatsoever on packet sizes or any other aspect of how the file I/O across the network behaves.
If there is a bug or inefficient programming, it is not ESRI you need to convince - it is Microsoft.