Quite often the estimated speed is rather pessimistic, but not in every case
Unlocking your modem or obtaining a spare one & unlocking it would give you stats such as the following:-
Max: Upstream rate = 4980 Kbps, Downstream rate = 34468 Kbps
Path: 0, Upstream rate = 4999 Kbps, Downstream rate = 29010 Kbps
Discovery Phase (Initial) Band Plan
US: (0,95) (868,1207) (1972,2783)
DS: (32,859) (1216,1963) (2792,3959)
Medley Phase (Final) Band Plan
US: (0,95) (868,1207)
DS: (32,859) (1216,1963)
VDSL Port Details Upstream Downstream
Attainable Net Data Rate: 4980 kbps 34468 kbps
Actual Aggregate Tx Power: 6.3 dBm 12.6 dBm
============================================================================
VDSL Band Status U0 U1 U2 U3 D1 D2 D3
Line Attenuation(dB): 8.3 54.3 75.3 N/A 21.9 64.2 0.1
Signal Attenuation(dB): 8.7 53.7 N/A N/A 21.9 64.2 N/A
SNR Margin(dB): 5.5 5.5 N/A N/A 6.4 6.6 N/A
TX Power(dBm): -4.4 5.9 N/A N/A 11.4 6.4 N/A
Clearly, 80/20 is no use for my longer distance connection as I can't even achieve 40Mb.
I have attached an example of a connection that achieves the full 20Mb US sync speed, but only 65.822Mb DS.
We can see that 20Mb US is achieved at lower than tone 2783 (approx 12MHz).
Some DS bitloading is achieved right up to tone 3959 (approx 17MHz).
You connection may be able to achieve most of the 20Mb US, but is perhaps unable to use much (if any) of the higher frequency DS tones, therefore your estimate is only 55Mb DS.
Without access to your own connection stats, particularly the MAX rates (attainable rates), it would be pretty much anyone's guess as to what you would achieve if switching to 80/20.