Here's a question: I saw a charging cradle that charges an iPad and iPhone simultaneously. If the iPhone would charge an iPad too slowly, would using an iPad charger on an iPhone charge it much faster than the standard charger? Would it cause any damage or premature failure to the iPhone's battery?
No, there is charging circuitry in the iPhone that controls the speed at which it charges. If it didn't, then the answer would be yes because too much heat damages rechargeable batteries.