Can I safely charge an iPhone 4S with a 5V 2A source? I just bought a New Trent IMP99D external battery, to keep my phone alive while gaming during long-haul flights. It has two USB ports to connect to, one which supplies 1 amp at 5 volts, another which supplies 2 amps at 5 volts.

The instruction manual says to connect smartphones to the 1-amp port, and any iPad 2's to the 2-amp port.

Is there any harm, or do I incur any inefficiencies, if I connect my iPhone 4S to the 2-amp port instead of the recommended 1-amp port?
When I travel, I only bring my iPad charger and use it interchangeably with my iPhone 4 without noticing any battery life performance and by that extension it should be the same for iPhone 4s. 

Apple's website seems to confirm this; For the 10W USB iPad charger - "It also charges iPhone and all iPod models with a dock connector."