networking - Realistic Network Load Testing

I am trying to benchmark an ASA under various conditions
but what is throwing me off is my baseline seems to be odd. I am trying to load an ASA
to full capacity. See the attached topology
diagram:

href="https://i.stack.imgur.com/kmRVP.png" rel="nofollow noreferrer"> src="https://i.stack.imgur.com/kmRVP.png" alt="Topology">

The players
are:

• C1 a Linux client
  runs a continuous download of a 300 GB file and loops this from S1, a Linux server
  running HTTPD.


• C2 a Linux client also runs a continuous
  download of a 300 GB file and loops this from S2, a Linux server running
  HTTPD.


• C3 runs AB to try and generate more connections.
  ab -n100 -c99999999 http://10.0.0.57/ This is to S3, a Linux
  server running HTTPD.


• Cisco ASA 5520 running
  8.4.

What I
found odd was that even with all this going on the max I saw was just over 500 Mbps
(observed via NLOAD on both VM box physical interfaces). Is this normal? Everything is
Gig. Some questions:

• Is it
  likely that my crappy Linux desk switch is
  bottlenecking?


• Does NATing really kill performance that
  bad or is something else going on? The CPU on the Dispatch Process was 30% under
  load.


• Is this is likely a disk issue as the servers are
  simply reading the file as fast as they can?


• What I found
  odd was that C1 would not transfer at it's full speed until I had it download 3 copies
  of the file from S1 at once (about 250 Mbps at this point). Why are 3 parallel downloads
  from S1 faster than a single download? Shouldn't S1 send as quick as it possibly
  can?

Is there
a better way to load test network equipment. Downloading a single large file does not
seem realistic. I am trying to simulate a busy network doing web things and load the ASA
to capacity.