The whole point of DSL was that it was designed to work over the existing installed based of far from perfect phone wiring that exists not only from the phone company to the premise, but within the premise. That would allow digital service to be delivered without the huge cost of a new infrastructure or modifying the existing one. The total length of wire can be as much as 3 miles, from the phone company CO or other eqpt, through the house wiring, to the DSL modem. All of that is just plain old phone wire. What kind of data rate you can get does depend on the gauge of the wire, the number of gauge changes along the way, any bridge taps, etc. On a short line without the above imperfections, you get the highest rate. On the longest line, with many imperfections you get a lower rate, or in some cases it may not work at all.
But like KRW, I'm having a hard time trying to see how changing out the last 50 ft of traditional phone wire and replacing it with something else, suddenly results in a 3X speed improvement. If the line length went from 3 miles to 1/2 mile, I could see it. If several bridge taps or loading coils were removed, I could see it. I'm wondering if the real problem was not something loose, eg a poor connection at the ends where the 50 ft cable was being changed out, and maybe the new cable made good contact where the other one did not. It would seem to me that if DSL were that sensitive to a piece of regular 50ft phone wire, the whole thing would not work, almost everyone would be having all kinds of problems, and they could never roll it out. IMO something unusual, whatever it is, was happening here.