You're right that the router alone can't explain the variety of results with all the gnutella changes and the dynamic nature of the network.

FWIW, I've always been on a static IP.
I did try a direct connection for a while, and didn't see much difference between it and when I was behind my NAT. I've seen the most improvement by fiddling with the firewall and NAT software.

Looks like LW 3 is getting closer, and will probably bring in the most changes for Macs, and probably other platforms too. There are still lots of GUI bugs, but the project seems to be emphasizing search changes right now.