How to determine MRF24WG0MA wifi bottleneck?
You can try tweaking the SPI up to 20M instead of the default 10M (WF_MAX_SPI_FREQ).  Setting the optimization to 3 helps too.  Also make sure you try the latest libraries.

Are you using iperf to measure the throughput or just monitoring file transfers?  iperf will give you more consistent test results.