I have not seen confirmation on a quick google but what is confirmed is it will not harm the device - which I knew anyway and I have charged my iPhone off of 2.1A chargers, but have not noted the charge time. Actually I had a USB cable that split into 2 and would frequently charge the steripen and iPhone at the same time, each rated for 1A charging.
I know there is a significant difference between charging at 1A and 500mA and want to make sure that you are getting better charge time at 2.1A compared to 1A charging and not comparing it to 500mA charging. Or in other words if the iPhone is truly limited to 1 A charging the 1A and the 2.1A will charge identically, but if you are actually comparing it to 500mA charging the 2.1 A charger will be faster (as it will be charging at 1A, compared to 500mA if the device is charge limited).
I'll give it a try.