The 15 watts is between the 23mm gp4000 and the 35mm bon Jon pass. I'm assuming the relative differences between the 25mm gp4000 and 38mm Barlow pass will be similar.
I'm not sure why you're surprised that a larger smooth diameter tire comes close to a smaller smooth tire. If anything the trends we've seen is that increasing diameter reduces rolling resistance. I would be interested to see the results of some of the more narrow compass tires, I think they have a 26mm version if I remember correctly. The reason very wide tires are not used in road or triathlon racing is that they would be aero dogs and force frames to be dogs as well. Even as it is, the aero drag increase of 25mm tires over 23mm ones generally outweighs rolling resistance gains especially at race speeds. Of course for those not racing wider tires have much more tangible benefits than fractions of a watt. 15 watts on the other hand is more than I'd be willing to throw away for the tested models other than the Snoqualmie pass.
I would love to see other data on these tires though as it's always good to confirm a single data point. I have hopes Jarno Bierman will start looking at these types of tires.