I was actually pretty surprised by the results! Figure 10 shows throughput vs. path loss for the six adapters running downlink. You can see that most of the adapters have similar curves, with the two exceptions being the Broadcom Intensi-fi based WPC600N and "high-power" EnGenius.
Figure 10: Throughput vs. Path Loss Comparison - Downlink
I repeated the test on the WPC600N to make sure that the results weren't a fluke, but obtained virtually the same curve on the second run.
The EnGenius' clearly superior result shows that it really is providing a performance advantage, even when receiving. The dipole antenna isn't providing any gain, so there must be some receive amplication being used.
The other performance to be noted is that of the Intel 2915ABG, which disconnects quite early compared to the other products. This could be due to a glitch in the ranging algorithm, since througput is still relatively high when it disconnects. Again, I made multiple runs with the Intel adapter with the same results.
Figure 11 shows the results of the uplink tests, which would reveal differences in adapter transmit performance. The same three adapters once again differentiate themselves from the pack.
Figure 11: Throughput vs. Path Loss Comparison - Uplink
The EnGenius once again has a clear performance advantage, not only maintaining higher throughput with declining signal level (higher path loss), but also remaining connected with significantly higher path loss. This isn't that surprising, since the adapter is using a higher transmit power than the other adapters. So for a given path loss, the signal received at the WRT54G would be higher.
The Linksys WPC600N this time stays with the pack during the "waterfall" part of its curve, but it starts out with measurably higher throughput while the signal is strong.
Finally, the Intel 2915ABG once again disconnects sooner than the other adapters and again at a relatively high throughput.
So there you have it. The most controlled test that at least I've seen, on the effect that changing a wireless adapter can have on performance. So if you have been thinking of trading up to a draft 11n adapter in hopes of goosing your wireless LAN's performance, better think again. Looks like you could be better off with a lower-tech and lower cost solution.