11Mbps is the max *signalling * rate for 802.11b. With the right packet size (and a strong signal) the good devices, Cisco included, will achieve about 6Mbps (the best test is an FTP file transfer).
At the lower signalling rates (1, 2 & 5.5Mbps) you should also get about 66% of that for data throughput. It's a little different because 1 & 2Mbps use a different coding, but that's close.
Actual throughput for 802.11a is about 22Mbps, with around 35Mbps for the proprietary 'turbo' modes.
Matthew Wheeler
Chief Wireless Architect
www.BlueModal.com