The U-verse Residential Gateway will try to work around the spectrum around an interfering signal. (More precisely, it uses "Digital Spectrum Management" techniques to find "bad" frequency channels and reassign their data bits to unassigned "good" channels.) I made a 100W test transmission at 3501 kHz which produced the following bit loading spectrum. That's after some delay - the refresh cycle is 5 minutes. It helps to quit the app and then restart it. This test was done with the improved filtering (Type 77 toroid on DSL input line).
You can also see smaller notches at 660 and 880 kHz, which are two New York AM stations that are fairly strong here. The big notch at about 2.5 MHz does not correspond to a strong RF signal that I can detect.
So 100W produces the notch seen here, and 500W is enough to kill the DSL session entirely.
The RG will "retrain" itself upon a reset operation (pushbutton or power up), or after disconnecting the input for a few seconds.
After rewiring the VDSL input, some apparent improvement in high frequency bitloading was apparent. (Why? We only added about 10 ft of Cat5 and the surge suppressor.) Note the new channels in the second download band, above 5 MHz. The "Max rate" according to U-verse Realtime has increased by about 10%, to over 33,000 kb/s.
Sometimes, the RG outsmarts itself into a funny state. This is what we got after an 80M QRO transmission. The FEC error rate was quite high, even after transmission stopped. I had to reboot the RG to get back to a usable bitloading condition.
After a session of testing 80M at high power, a big gap was cleared out of the band.