Hmm hard to say, my 1080 FTWs (one DT) seem to run a bit cooler than my GTX 980s did, but the 980s had smaller fans that the FTWs do, and I've also swapped some case fans since then too, so not a really fair compare, but at least the wattage is about the same 175w for the 980s and 180w for the 1080s (I know the ftw version has a bit higher max, but not sure if that has much effect on if it reaches that.)
SO I guess what I'm saying is... I don't really have a good measurement of it either.
my temps are at 53c for bottom card, 68c for top card, with the default "custom" fan curve that is done by Precision X OC (which is more aggressive than the default curve by a fair bit.)