Like any DSL technology, you are limited by distance. The higher frequencies will not travel as far... so if you are distance-limited, then the change of profile (alone) will not increase your speeds.
However, it looks like BT are rolling out a change in bandplan (from 997 to 998) at the same time as the change to profile 17a.
The main differences are:
- In the existing frequencies (under 7MHz), around 0.6MHz is swapped from upstream to downstream
- The first 1.5MHz of the new frequency allocation (above 7MHz) is given to downstream; if they stuck with 997, it would have gone to upstream.
This means that, if you are distance-limited, you will still see a re-arrangement of your upstream & downstream speeds - which could be a gain as high as 5Mbps on downstream and a similar loss on the upstream (but probably only if you are on a package with a 10Mbps upstream)
And where you aren't distance-limited, you will still be limited to the 40Mbps that you've bought from the ISP. The jump to 80Mbps does not appear to be free, and will need to be ordered from your ISP.