cmb,
The limitation is probably in the endpoints (Vostro 1520 laptops). Using the same iperf test ("iperf -s" on 1 laptop and "iperf -c x.x.x.x -t 30" on the other laptop) with a crossover cable between the 2 laptops only yields an average of 172 mbits/s. I did the same test thru the following firewall distros: vyatta core, astaro essentials, & Untangle. Here are those results:
Laptop -> Laptop via Crossover cable
172 Mbits/sec
Astaro
Intel -> Intel = 165 Mbits/sec
Intel -> Broadcom = 147 Mbits/sec
Broadcom -> Intel = 132 Mbits/sec
Broadcom -> Broadcom = 140 Mbits/sec
Vyatta
Intel -> Broadcom = 114 Mbits/sec
Untangle
Intel -> Intel = 165 Mbits/sec
Intel -> Broadcom = 160 Mbits/sec
Broadcom -> Intel = 200 Mbits/sec
Broadcom -> Broadcom = 200 Mbits/sec
Note: the NICs in use also made a difference. When I did the pfSense test, I was going from Intel -> Broadcom.