Are you getting similar speeds now ?
A speed test from speedtest.net may give you an idea of how close you are to the advertised speed.
You may want to try the TCP Optimizer, just choose your rated speed, and apply the optimal settings. Tweaks for WISPs (Wireless ISPs) are pretty much the same as conventional wired broadband connections. There may be some additional complexity as to pointing the antenna, using proper connectors/cabling and a clear channel, but that should be handled by your ISP. You may also want to keep the modem/router away from any possible sources of EMI, as most of them are not well shielded against interference.