I personally don't have an OG stock charger, so I don't have any experience using a charger rated specifically at 1.2A. I only have USB chargers rated at 700mA, 850mA, 1A and 2A. Plus, an assortment of other power supplies rated anywhere from 2A to 30A. Of course, I do see a faster charging when going from 700mA charger to 1A, and then to 2A, because the first two are under-rated compared to the phone's specs. However, once I go with 2A or the higher rated power sources, there's no difference at all.
Hope that explains it.