Color me shock by I never thought it was possible to passively cool a Reference R9 290x without resorting to a full custom water loop.
I have the PowerColor R9 290x PCS+ which is basically just pre-OC'd reference with a custom air cooler. I discovered after replacing the H80i GT 120mm AIO because of pump failure on my Corsair HG10 with Cooler Master's Nepton 140XL and configuring fan curves for it using SpeedFan that it was being cooled relatively well without the fans spinning.
Obvious reasons, much more efficient, or, at the very least, a working pump compared to H80i GT. Also, much bigger radiator helps by a big margin. SpeedFan doesn't rev up the fans for the rad until near 60°C, the blower fan on the Corsair HG10 has a custom fan curve set at 10% until near 60°C as well.
Things to note though, I don't like the increase in idle temps. Without the 0db fan mode, it idles at 43°C with rad fans at 33% which is the lowest the Jet-Flo fan will spin. With 0db mode, it idles at around 51-55°C. However, the pump speed is set well below 7v, and this pump is meant to run at 12v, but I undervolted it for quieter operations because it does have an annoying whine noise.
Compared to the semi-passive cooling mode of Sapphire's Vapor-X 290X, it's really not all that different. HiTechLegion does have have lower idle temps here, but it's open air test bench. It does however speak volumes to the level of engineering that Sapphire is capable of, in that, to mach their cooling capacity, it took for me to use a bulky hybrid solution.