I think you're confusing volts and amps. Higher amperage wire has thicker copper conductors. Higher voltage wire has better insulation and some standards about how far connections have to sit. Raising voltage increases the chance of sparks, but amperage is what runs into heat problems with modern insulation. Wattage (power) is simply Volts x Amps. To get higher wattage, you can increase volts or amps (or both of course), depending on what materials and devices you have available. The whole point of going to higher voltages in usb is to carry more power without making the cables thicker and without overheating thin wires.

Most circuit boards are 5v. I'm sure Apple already has a converter on board. Also, the DC conversion problem is outdated - both on difficulty and inefficiency. Solid state devices are able to switch power on and off at a frequency to make an onboard converter coil work with the same efficiency as AC. The little extra heat lost from the switchers is made up for by not wasting time/energy on phase cycling. Check out "buck buck" converters.

Volts, amps, and watts are all different units and do not interchange. They are related, but serve different functions.

Fun fact: powerlines all use much higher voltages on the transmission wires than what you have in your house. The lower amperage (but same wattage) uses less material for wiring and loses less energy as heat along the way. North American houses have 120v, the transformers on the nearest telephone poles drop it from 440v, and the overhead lines are 440,000v with various possibilities in between substations