PROBABLY, because of power limit.
I am waiting on a new PSU to test this theory (one that has different voltage regulation, to be sure it isn't VDROP protection).
I never saw the 380X throttling due to temps, in fact its behaviour regarding temps is bat**** retarded (it will just happily keep increasing temps without increasing fan speed or throttling until I am ****less scared with the crazy PCB varnish smell and coil whine, thankfully using Sapphire Trixx to setup a decent fan profile fixes it, even "power virus" tests make the temps top at 60C with fans at 60%).
Gigabyte 380X models were confirmed to have power limit problems, they would stutter like crazy if you used stock bios, because the stock bios didn't had enough limit to hit the stock clock even at low usage.
With my Sapphire 380X, setting the power limit to +20% fixes the stuttetering for most part, but not 100%.
leaving power limit on 0% make the card sometimes be unusable, with Crimson 16.4 it was just ridiculous, GTAV would look like a slideshow and the GPU would run at 600mhz.
With 16.6.1, GTAV runs on average at 920mhz with power limit 0%, and runs at 1040mhz (the advertised clock from Sapphire) at power limit 20%, with the occasional stutter in game (and in out of the game, looking at graphs, it is still throttling frequently, but stay at 1040mhz for most part).
Also I posted in other thread about this, but reviewers discovered during the MSI and Asus 1080 "review-only BIOS" that the review-only BIOS had modded power limit.
I suspect this might have happened on AMD GPUs too, because it would be hard to detect (how a reviewer would discover the power limit is higher, specially when the control panel just display percentages, and not raw values?)