You will need to enable SNMP and use a piece of free software such as MRTG to see what's happening on your PIX interface.
Give it a day or so while you're heavily loading the link and then you should be to extract some useful data.
Remember that although the link is capped @ 20M but this is between your CPE and the ISP's edge device (probably a switch in this case) but this doesn't mean that they will secure an uncontended 20M on their uplink ;-).
Run the tests, save some graphs and if you still believe that you should get more then send them to your ISP and let them investigate.
We have configured the outside and inside Interface with official ipv6 adresses, set a default route on outside Interface to our router, we also have definied a rule , which also gets hits, to permit tcp from inside Interface to any6.
In Syslog I also se...