In my wife's car we have a griffin car charger, this will charge the phone slowly while wazing.
My car has a Dension IceLink fitted to the head unit which makes it think that the phone is a CD changer. It charges the phone faster than the mains charger supplied by apple even when wazing. I can Waze for a 2 hour trip and it will add about 60% to the charge in that time.
Well, this does not exceed the results of your home charger, but it reaches the same level.
Again: The iPhone decides itself, how much current it draws from the charger. It can recognize a dedicated charger and even determine if the charger is on-par with the iPhone charger or the large iPad charger. If the manufacturer of a dedicated charger copies apple's wire signaling, the result will be identical to that of the original equipment.
On the other hand, even a 5A-generic charger without the signaling will only charge at 500mA because the iPhone will not draw more current. The iPhone knows three speeds: <500mA for non-dedicated chargers or USB-ports, ca. 1000mA for chargers with iPhone coding and more than 1000mA for chargers which signal iPad-compatibility.
Thus, you can charge reliably during navigation with a small charger if it's wires are coded correctly.