(Reminds me of how it only gave me 10Mbps mode when I connected my Lenovo Z460 directly to my SCPH-10000 D: )
But there's something that bothers me... OPL doesn't have DHCP enabled, does it? So why are you trying to identify the IP address of your PS2? Shouldn't it be a static address?
And for some strange reason, I noticed that the MAC address of my SCPH-39006 didn't change with the new driver. So perhaps I made an error somewhere or it was with a different console. :/
(Perhaps I really was indeed mistaken about the MAC address changing... but I really thought that I saw the MAC address of one of my consoles change to show that Multicasting was enabled, and that alone prevented my PC from sending frames to it until the old ARP entry was removed/refreshed)
The performance of the OPL HDL Dump server is still dismal though: 0.8MB/s for me, which is quite expected. TCP is really too heavy for the IOP. >_>
Perhaps it's due to traffic shaping by your router? I noticed that HDLDump will use a random port, and sometimes it might end up using ports that are used by other network services as well.
Since UDP doesn't have a concept of connections, I wonder how it's deciding on what ports it can legally use...