The longer the cable, the more voltage drop, not current drop. And yes, if the device pulls the same current that lower voltage will reduce the wattage slightly. (And charge speed is not current. It’s power: IxV) However, voltage drop in a cable that is of normal length isn’t going to be an issue, and putting a higher wattage brick behind it isn’t going to do squat.

If a 20W charger is used, the puck would still attempt to draw 15W of power because that’s the rated charging load. The brick isn’t going to shove more power into the device to make up for voltage drop. (And if it did, it would be as a result of higher current, which would also increase voltage drop further).

You decrease the effects of voltage drop by either increasing the conductor size or by raising the source voltage (and thereby decreasing current for the same load).