Well, I would ditch the backfire antennas, those things are just about worthless. The pac-wireless 2ft solid dishes are inexpensive, and work quite well (other real dish antennas are quite good as well).
After changing antennas, I would test modulations (data rates), one at a time. If you start to see latency fluctuations (or unexpected registration drops), you have gone too far. I would then back it down two steps from the point you start to see trouble, and lock down both the basic and supported rates. With luck, you may not have to back down from 54Mbps at all.
5mhz and 10mhz channels may be worth exploring as well; as you may be able to get just as much throughput, with a better signal level (+3dB for 10Mhz, +6dB for 5Mhz), and less spectrum usage. This is especially appropriate if you are only trying to get near 7Mbps, 54Mbps @ 5Mhz should easily net you 6.5Mbps real throughput, if you can keep your signal clean and strong enough.
I would not use Nstreme, since the CPUs in those RBs are a bit on the weak side. Disable connection tracking, don't attempt to do any queueing or firewalling, and otherwise strip down your configurations as much as you can.
Also, be sure you are not testing to or from the radios when you do your speed tests, only go through them.