I've set up basic bandwidth control and find it works well. I haven't got into QoS yet as I don't have a specific use for it at the moment. If you want to bandwidth manage on a per user basis, what about creating PPPoE tunnels from each user? This essentially means each user has their own interface to apply policies on and you get true authentication.

I've found RF amps add latency although as mine boost the rx as well it could have been a noise issue. What about something like a Ubiquiti SuperRange card? They have a stock output of 400mW and rx sensitivity of -97dB IIRC. Saves a bit of money over using a separate amp. WISP-Router.com sells them for about US$120.