I am using the standard conf: --lookup-gap 0 -g1 -I 8 --worksize 64
I have custom modified my bioses with a HEX editor to get low Vcore values and the card to still boot. But I think with Sapphire cards you can go pretty low using Trixx also.
Here is a list of Watt/Mhash/Mhz/Vcore's I am using:
4x 17,3Mhash @ 950Mhz/0.856V = 405W
4x 18.3Mhash @ 1000Mhz/ 0.880V = 445W
4x 19Mhash @ 1040/0.900V = 460W
4x 19.8Mhash @ 1080/ 0.920V = 480W
I am not using higher setting such as 1150Mhz because it demands too much voltage
What about memory clock?
I use 1000Mhz
Yeah, I use 900 but no difference with 1000. For some reason turning it all the way down to 750Mhz actually starts taking more electricity

Those core/voltage values are amazing. How is that possible for you?
My 4 x 280x rig can only go down to 1080/1.054v @ 19.8 Mh/s = 585W.
maybe is wattmeter is giving non-correct wattage
I doubt it as it seems to be accurate to at least 5W.
Are you able to post a screenshot of GPU-Z sensor tab on one of your 280x while your rig is running?
I'm very curious what the VDDC shows via the sensor.