Comment 104 for bug 124159

Don't expect much progress on bcm43xx since it has been deprecated for new driver:
http://linuxwireless.org/en/users/Drivers/b43
http://bcm43xx.berlios.de/

Generally speaking don't expect much from these cards as Broadcom keeps the documentation secret and the native drivers are 100% reverse-engineered. And don't expect much from Ubuntu: they can do little more than packaging and shipping the code of kernel developers.

This aside I confirm that on my 4306 reducing the bitrate usually gives better stability and as a consequence a better throughput.