I know it's a little different scenario than the original question, but in long rides I've seen my "watts lost" from drivetrain/frame go from 2% up to 8% as the lubrication is lost on the chain. I have both P1 and powertap GS hub powermeters and in a recent ride I started with an 3w difference which about 6hrs in grew to 14w. And this was riding at a lower power around 170w. At higher wattages it is even higher. Seems to be a percentage loss rather than a fixed wattage.