Fast charger for smart phones

Correct from an electrical perspective, but the device may not draw all 10 watts. A more accurate way to phrase it may be that your 5 watt charger may only charge your device half as fast as a 10 watt charger. Modern smart batteries have protections that prevent them from being charged too fast. Additionally, lion/lipol batteries will only rapid charge to about 50% then they begin to back off how fast they will accept charge in order to preserve battery lifespan.

iPad is an interesting case. When I hooked it up to an ammeter, I noticed that with the stock iPad charge/sync cable, it was not drawing 10 watts, but only 7 watts. I then constructed a cable with thicker wires and was able to see a 10 watt charge. Why apple does not up sell a premium cable, I have no idea. They are perhaps throwing away 30% of the power as heat loss, not very green.