actually, I've been able to get a ping -f -l 8972 through the dgl-4300's switch then through a dgs-2208 switch to another computer. however, I turn off the fragmentation option on the 4300 itself and let internet traffic negotiate itself to the standard packet size (ping -f -l 1480 works to the gateway part of the box.)
I am using the latest firmware available as of this date.
nrf