I finally got around to doing some more tests on the PS3 internet connection. It turns out that if i plug the PS3s directly into the cable modem the new PS3 gets connection speeds as good as or better than the old PS3. So there's something weird about the connection through the router that the old PS3 can handle but the new PS3 can't.

It's rather dumb that it's the new model that's having problems, but given that it's an 8 or so year old 10/100 router i really ought to just bite the bullet and upgrade. As soon as i can figure out a good new model that communicates with the PS3 and Wii happily that is.