While most engines have the primary break-in done at the factory, UOAs of engines that have been switched to synthetic early consistently show a prolonged period of higher than normal iron, copper, brass, aluminum, and other materials that indicate the engine isn't fully broken in for a few thousand miles. Does it matter? Probably not since the most important part of the break-in, seating the rings, is already done for us.
5k to me is a good number to switch. I have switched engines I have built to synthetic at 1500 with no ill effects.
I too have thought about switching to 5w-30 (the 2.0 and 3.5 Ecoboost's use this) but I think during the warranty period, I will stick with 5w-20.