Hey dude, had the exact same problem.
I literally just fixed it right now. Only problem is, I was doing two things at the same time (registry changes and driver I found), so I don't know which one fixed it :P
here is the driver I installed, then rebooted and changed to AHCI...
Apparently there are great gains. Guy on OCN, mins and avgs up higely in Far Cry 4 at 1440p. Avg of 65 to 80!
http://www.overclock.net/t/1547314/official-amd-r9-radeon-fury-nano-x-x2-fiji-owners-club/2430#post_24147663
The Asus Matrixes were over volted a ridiculous amount, massively inflating the power draw But still, the power draw on this 290x is quite a bit lower. I've read on ocn that it's due to th memory they use or something, either way, it's know that the Powercoor PCSs draw less than other 290...
This card is a beauty, I have the 290 version and have it clocked at 1200/1600 at plus 100mv (very reasonable long term overclock, spikes to 1.3v but stays around 12.5 most of the time). and at 50% fan it stays below 75c in all games.
The 290x Vaporx has much better power delivery compared to...
Both the PCS+ and Trix have very good aftermarket coolers. The PCS+ scores better in terms of temps in benchmarks, but this is primarily due to the fan curve on it at stock (very aggressive). Outside of the Vapor X and Ligtning coolers, the PCS and Trix are the best 290(x) coolers.
Ah well. I suspect the VaporX is a slightly beefier cooler, and probably has fans which spin faster than 2000 RPM (a limitation of the Acclero, the fans even at 100% were really quiet, why not just put fans in there that can hit 3000 plus RPM and let the user decide? :P).