The PN code is adjusted depending on how much corruption resistance is required, from what I can tell most vendors base their connect speed on the RSSI (Receive Signal Strength Indicator) which is hash of actual signal strength, missed beacons, re-transmits due to corruption etc. I have found the best way to check the quality of a link is to do a large FTP file transfer and move around with a laptop using a bandwidth metering program. The max throughput you will get on a 802.11g link is about 22Mbps, as you move away it often says it is connected a 54Mbps but the actual throughput drops with re-transmits and maybe larger PN code (if it adjust dynamically). There is no standard for how vendors measure their RSSI and some seem to want to hold on to that 54Mbps connect rate even when it should be lower for a more reliable link. Most likely for marketing purposes.
Is this any help to you?