@bradyr: I would have to agree your assessment. I would say 50 - 60% is about what I see using 20mhz channels. Sadly in 2.4ghz range using 40mhz channels didn't seem to do anything for me. (nothing in nearby channels either)
@JeepMatt: IIRC the numbers for just overhead from encryption are something like 50% WEP, 30% WPA, and 20% WPA2. A new encryption standard might be able reduce overhead a bit more in the future. So Not only are you getting stronger security you are now seeing the extra bandwidth from using WPA2.
@GmDude66: It should be able to do 300mbps but I think it's only in 5ghz and not 2.4ghz. I know that a 2008MBP can do 300mbps on 5ghz, and I'm pretty sure it wouldn't do it on 2.4ghz because it wouldn't run 40mhz channels. (Well a bit less than that because they don't seem to use Short Guard, probably disabled for power savings) The late 2008 Macbook has the same BCM4322 chipset as the MBP.