virtualbox.org

The '9.8.3. Tuning TCP/IP Buffers for NAT' part only sets buffer size for the default 'NAT' net mode,which goes under the 'DisabledModes' in vm's xml when using 'NATService' custome network.

I don't find any doc on how to set the buffer for NATService type.
I thinks this is possible? Since it's still NAT and both goes through vboxnat.exe

FWIW, the VirtualBox "NAT" and "NAT Network" networking modes are completely separate implementations. When using "NAT Network", VirtualBox uses the underlying mechanisms of the host OS, especially the socket options. Only if the host OS is Windows and the send socket buffer is smaller than 64 kB, it's set to 64 kB.