I have a sapphire r9 270x OC edition, by default it turns at 1070*1400 with a voltage of 1238, I used the oc programme of sapphire named trixx to OC this GPU to 1100*1500 with the same voltage of 1238 and power limit on 0. I changed the fan speed to 20c/20% 40/40 50/50 65/70 80/80 85/90 90/100 turns at an average temp of 68c playing bf4 for more than 100 hours and no problems so this seems like a save sweet spot to me.
Just for the information : if you want to play bf4 in multiplayer don't put the video settings on high of ultra but keep them on normal, the r9 270x card can handle the ultra but you will probably be slightly slower in observing and shooting your enemy. And to be honest, I can hardly experience the visual difference between normal high and ultra.
psu 550w, cpu amd 6300 fx BE OC on 4400ghz cooled by coolmaster, memory 8gb, Windows 7 64b, playing BF4 64b,