For me it did mean a significant improvement. I was on 10Mb L and my service had been deteriorating for some time, only getting full speed after midnight and at peak times it would drop to between 2-5Mb.
After the speed doubling it got a little better but never maxed above 18Mb and again would drop to below 10Mb at peak times.
My PSU failed for my Ambit 256 modem and they sent me a SuperHub (I'm one of the lucky ones having no prob with the SHub!). Immediately I was getting improved speeds on the 20Mb config, within a week they moved me onto 30Mb config and I seem to get over 30Mb whenever I test, even at peak times, a typical test result taken a few mins ago is:
As Kwikbreaks says you'll eventually get 20 or maybe 30 for free anyway like I did!!