I don't have any issues with performance that i've stumbled across. I am not sure what changed in the GUI in terms of additions, removals, or change in default settings, but you may want to compare the advanced wireless of the old firmware to the new firmware. For example, i usually acheived 160 - 270 Mbps throughput with the router laying horizontal close to the floor sitting on a speaker, with my laptop in the same room.
I just recently cleaned up the living area and brought in what I can best describe as afour foot tall floating shelving unit (no tops, sides or backs), put the gateway on the top shelf, this time standing vertically with the antennas pointing up left and to the right. I was able to maintain 270Mbps from the far end of the living area about 10 meters. I then logged into the router, enabled short-GI and was only then able to acheive 300Mbps throughput, though the connecton quality is only at 70% or so, using the dwa-160 using the dongle. Another wireless client who uses a 108Mbps Linksys usb card see's a constant 108Mbps from about 8 meters away through two walls, in his bedroom. My bedroom is farther and usually see anywhere between 18-80Mbps fluctuating. Havn't tested it with this new setup yet.
So what type of performance loss are you seeing when upgrading to the b05 firmware?