You sort of answered your question with your statement.FWIW, Speedtest doesn't show sustained throughput.
care to explain? seems pretty sustained to me (for short term anyway).
Speedtest and similar sites run a short duration test to check your bandwidth. Just like USB vs. Firewire, short term transfers are not necessarily indicative of long term (larger) transfers. File size, duration, distance to your ISP (ADSL), distance from the source, the server, etc. can affect your actual throughput.
For fun, download Mac OS 9.2.1 from Apple's site. The file is 82MB in size. Dial up used to take about 6.5 hours. Typical ADSL around here is 2-3 minutes. FIOS is around 1-2 minutes. My friends FTTH was 7-12 seconds.
Of course the infrastructure is continually improving to support our increased thirst for speed.
On day we'll look back on my friend's FTTH connection and reminisce on how slow it was.