There is more than one place to set the packet size for a give vision system. The one you’ve set via Management Console (or GenICam Browser alternatively) might be the one stored in the GenICam.ini file. This is a setting only for the CVB driver. You can check the file by browsing to %cvbdata%Drivers (on a Windows system).
In the Management Console, it is set here:
In GenICam Browser these can also be found via the settings menu at the right side of the application.
The setting you are querying via the nodemap is most likely the one of the camera itself. It can be set via the GenICam grid (either via Management Console or GenICam Browser) for instance.
Then, there might also a packet size/jumbo frames setting for the NIC and for the switch (if you are using one).
In regard to why setting the packet size to “8704” does not work, this might be because not every arbitrary value can be set here. The values usually can only be set in fixed steps. Also, the value might not be changed when the grab is active.