I run 2x Sapphire Tri-X 290Xs at 1100/1625 (6.5ghz), stable and works great. Using the TRIXX app, I use VDDC Offset of 100 and power limit of 15. Don't remember what that translates to in terms of vCore, but from what I saw on the HardOCP overclocking article, it seemed 'mild'. Their 'Sane Voltage' setting was 150 VDDC Offset and Power Limit of 50. No issues with throttling and is completely stable.
It has been a while since I did it, so I don't remember how much more headroom there was if I continued to raise it, but I was happy with those numbers and left it alone.
Anyway, this is an interesting thread regarding underclocking it. I might just do that since I get way more FPS than I need right now with my setup. I just peg the VSYNC limit all of the time. I don't think it'll hurt me to drop some voltage and clocks and see how it runs.