I thought that all the VDSL2 services used the same frequency range and what actually changed was the bit loading. A 40mbps link can still be putting bits into the highest frequency slots. I suppose it might be that on some medium length lines there was no bit loading in the top frequency bins at all and going to 55mbps might mean some so far unused bins now have some bit loading.
When the modem has tones available for use on, say, a service capped at 40Mbps but the line is capable of an awful lot more, I'm pretty sure the filling algorithm will choose to load all tones to a low bit-loading, rather than highly-loading some and leaving others empty.
That means any line that gains from 40Mbps to 55Mbps will do so by extra bitloading on tones that are already transmitting. In digital terms, this gives the receiver a harder job (it has to distinguish between more/closer amplitude and phase options). But in analogue terms, the transmitted signal will still be of the same order: still a modified sine wave, still the same power.
I think the end result will be similar crosstalk.
I've just checked a couple of lines on MDWS, which are currently 40Mbps but have capabilities of 80Mbps. They all use the full set of tones, but a lot of D3 only uses 2 bits/tone.
The filling algorithm gets affected by upstream power back off, so U1 can get left with unloaded tones, while tones in U2 are more heavily used.
If upstream speed caps were being increased, then you might well see more crosstalk there.