There’s a common misconception that ADSL2+ is faster than ADSL2 on any line.
That’s not really the case. In simple terms,

ADSL2+ utilises twice the frequency range available on your phone line that ADSL2 does.
This again, in simple terms means twice as fast BUT that is only seen on short low attenuation lines.

If your line is only capable of supporting 7meg on ADSL2 then it’s only capable of supporting 7meg on ADSL2+
as it can only usually allow the use of the same frequencies for both (see below).

However, if you’re lucky enough to have a line that can support higher frequencies then you get up to :
 
12meg on ADSL2 (the maximum possible)
but up to
24meg on ADSL2+.

The cross over between ADSL2 and ADSL2+ is therefore in the 10-12 meg range (typically 35-40db if the line is relatively noise free).

It can give faster speeds but usually only on short lines as explained above.
The only time that wouldn’t be true is for a moderately short line
(that offered some higher frequencies above those usable by ADSL2)
that had induced noise at the lower frequencies and was clean at higher frequencies,
in which case ADSL2+ would possibly be better as it could use those higher frequencies.

There is also the possibility that a network uses equipment whose firmware works better in
certain conditions with specific ADSL modes hence why it is mentioned G.DMT sometimes being
better for problem lines.