Well, surely it can be useful to use the Kill-A-Watt, right? For example, just plugged my iPhone 5 into the iPhone 5 charger and the Kill-A-Watt says that it's using 4.7W. Whether all of that wattage is being transferred to the iPhone, we don't know, I suppose, - BUT there's a max of 4.7W draw.
If we then plug an iPad charger into the iPhone 5 and see the same 4.7W draw, then we know that the iPhone is not pulling MORE than 4.7W, right?
Let's test that theory...
Just plugged my iPad 10W charger into the Kill-A-Watt. It's still reading 4.7W power draw when I plug my iPhone 5 in.
By my logic, this puts this whole debate to rest, no? We know that the iPhone 5 pulls up to ~4.7W. Certainly not MORE than ~4.7W.
I'm not a power expert, but the above makes sense to me in my head.
You make a faulty assumption that the AC/USB adapter is always pulling the same amount of wattage. It has already been pointed out in this thread that that isn't true.
The phone's built-in battery charger is a lithium cell smart charger and, in order to maximize the life of the lithium cells, it will sometimes draw much more power (for example when the battery is 30% charged state) and sometimes a lot less (for example when the battery is at 95% charged state). In most cases for lithium the charger will charge at a rapid rate up to about 80%, then switch to a slower rate until 100%, then either switch again to trickle charge or toggle on/off periodically.
To make matters worse, we're talking about the actual state of charge, which is different than the displayed state of charge, which is all lies. For example, when the display says 0% and the phone turns off, there's probably 10-20% left. Lithium batteries die if they are completely discharged and cannot be recharged, so undervoltage protection circuits cut off the battery. Likewise when the display reaches 100%, the battery hasn't actually reached 100% and will still continue to slowly charge until it does reach 100%.
This is why the only surefire way to know if it is faster or not is to drain the cells, start a timer, and do a full charge. And while there might be slight differences in the state of charge when drained, it won't be enough to mask a claimed 3/4 speed difference.