what are the manufacturer(s) and model(s)?

For Sapphire 280X Dual-X im using:  -I 13 -g 2 --lookup-gap 2 -w 256 --gpu-engine 1000 --gpu-memclock 1500 --thread-concurrency 8192  
and getting 345Kh/s per card. When increasing the gpu clock by incriments of 10, it falls off a cliff around 1020. At the stock speed of 1050 it performs very badly.

For Sapphire 280X TOXIC im using:  -I 13 -g 2 --lookup-gap 2 -w 256 --gpu-engine 1050 --gpu-memclock 1500 --thread-concurrency 8192
and getting 375KH/s per card. Stock gpu speed of 1100 is 'way off the cliff'. I think 1060 runs a bit faster and 1070 it falls over, so I like 1050. And on the memory, the Toxic's run at 1600 default, but at 1600 you can only run the gpu up to 1000 for 355kh/s before it tips over, so 1500 mem clock seems better for top speed.

For Gigabyte 280X im using same as Toxic, but lower gpu to 1020 for 365KH/s. Go much higher on gpu, and it tips over.

MSI 280X I still have running single thread (-g 1), as that's how I first found how to get my cards running decent and stable on Vert. But that only gives 300KH-ish, so I'm guessing that when changing to -g 2  the gpu clock will have to go down from its 1020 stock speed to like 980-ish for top hashing speeds.

EDIT: My MSI's are doing 365Kh each at stock speeds 1020/1500 (no need to underclock these guys I guess...) with TC 8192 (add or subtract 1 or 2 if you prefer)

So then add any other switches you normally use for fan and/or temp control, etc.