We had a similar problem we do large downloads and crank out about 1.5Gbps through our 6800's.. Make sure your using a performance HTTP profile not standard.
the standard profile inspects in/out packets where as performance HTTP only inspects inbound and doesn't really care what goes out. Depending on what irules you use fastHTTP might not work with some of your irules.
If you run in fast L4 you'll notice almost no loss of speeds. in performance L4 to a single gigE server we hit 105MB/s but in performance HTTP we only got 40MB/s at best.