Pls, provide some proof for those numbers. The 'under 5W mark' gets often claimed but i still have not seen a valid proof (a simple measurement with a wattmeter) of it other than some spec sheet.
The overhead does not matter really. USB 5V power supplies are cheap and efficient these days, yes you need to look out for an efficient one but even one with only 50% efficiency (which is really really bad) would only add 1W to the (lower than) 2W power in idle. That would still result in lower power in idle.