I recall having a similar issue when I first had FTTC, nearly 4 years ago.

Back then, the linux issues came down, IIRC, to some of the plugins in the browser. Firebug, I think, gave me bad throughput problems, even when it wasn't running. It wasn't the only thing though.

I too ended up limited to roughly 20Mbps.

Well I tried with all extensions turned off and I also tried with a new vanilla profile, all to no avail no. Nothing I have tried has changed the low speed using a flash tester.

I have also found another tester called http://speedof.me which is also not flash based.

Stuart