BT has found success in the lab by upping the power, adding more bits/carrier, and modifying the Power Spectral Density. The standard is being revised and the chipmakers are working furiously. Blue and green lines in the chart project the results from a model by Daniel Hincapie and Mathias Leibiger of Fraunhofer, presented in an EE Times article. A map is not the territory and simulations are not working chips, of course.
Their simulation projects an improvement of 150-200 megabits at loops under 100 meters, typical in apartment buildings. Lucky tenants will get a gigabit, a crucial goal for AT&T. They will be selling G.fast as part of their Gigapower offering, so they needed the gig for marketing purposes.
At 300-350 meters, BT's target distance, the newer units may raise speeds 20-40 megabits. Above 400 meters, there is little difference expected.