Not really answering your question, but a solution to sidestep this whole issue:
Use two wireless routers (or two in each location),
with one serving N clients and the other serving g/b clients.
Then the N clients won’t have the severe slowdowns at all.
Another tip, don’t use TKIP, stick with AES.
See <a href=”http://www.smallnetbuilder.com/wireless/wireless-basics/30664-5-ways-to-fix-slow-80211n-speed”></a>