It seems to be an "old" wives tale that, when using a wireless-N router in "mixed mode" to support legacy 802.11b or 802.11g devices, the performance of 802.11n clients will suffer.
Some places claim that when running in mixed-mode, all (some?) N clients run at G speeds. Others make the same claim, but say it only happens when a G client is connected.
Other places say N clients run faster, but still run about 30% slower than if the router were in N-only mode, even if there are no legacy B/G clients connected.
Still others claim there is no speed drop for N clients when running on a mixed-mode network. They say the only issue is that overall network throughput will be lower, because only one client can be transmitting at any time, so some of that transmission time must be shared with the legacy B/G clients running at lower speeds, reducing the overall throughput from what it would be if there were only N clients connected