The results I obtained when mixing 11g and draft 11n clients on the 854T were similar to those with the D-Link DIR-655, but with lower 11g throughput in some cases.
The test consisted of setting up a second notebook with a Linksys WPC54G V3 11b/g card (Broadcom chipset). Both it and the Netgear WN511B were associated with the 854T, which I set to 300 Mbps mode. I then ran two IxChariot throughput.scr streams, alternating the STA that got on the air first.
Figure 18 shows how uplink throughput was shared when the 11n pair started first, while Figure 19 shows the 11g pair starting first.
Figure 18: Mixed 11n, 11g STAs - Uplink, N starts first
I once again saw with the Cognio Spectrum Expert that the 854T and WN511B operated with a 40 MHz channel whenever they were on the air, regardless of which pair started first.
But you can see in Figure 19 that best-case 11g throughput is knocked down to about 5 Mbps from its normal 22 Mbps or so (seen Figure 21), or about 75%. And while the 11n pair also takes a throughput hit, it's only about 40%.
Figure 19: Mixed 11n, 11g STAs - Uplink, G starts first
I also ran these tests in downlink and saw similar behavior, but with only a 50% throughput reduction of the 11g STA (Figure 20).
Figure 20: Mixed 11n, 11g STAs - Downlink, G starts first
I should note that I also tried these mixed STA tests with the 11 b/g/n Mixed mode Wireless Optimization Setting enabled and didn't see any noticeable difference in performance.