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In a new rig, I got two 280x (Gigabyte R9 280X REV2 OC 1100MHZ 3GB 6.0GHZ GDDR5). I'm getting about 630 Kh/s each, and from what I read everywhere 700-750 Kh/s is achievable with a few setting changes.

I'm using the recommended 280x settings provided by guiminer-scrypt, so I think it's really a matter of overclocking gpu memory and cores and underclocking voltage. Has anyone tried? Could you share your OC settings?

Thank you.

try this:

 -I 13 -g 2 -w 256 --thread-concurrency 8192 --lookup-gap 2 --gpu-engine 1035 --gpu-memclock 1500 --gpu-powertune -20 --expiry 1 --scan-time 1 --queue 0 --gpu-fan 75 --lookup-gap 2 --no-submit-stale --worksize 256

Thank you. Unfortunately I'm still in the same ballpark. I wonder if the gigabyte cards ignore the overclocking settings. Would you recommend I set them in Afterburns?

With cgminer running hit the g key and see what it reports for the clock speeds.

Also you might need to go higher on the mem overclock to get those speeds. I have 3 7970's (same as the 280x) that need to have 1700 on the memory. two will hit 788 and the other will hit 750.

You really just need to over clock your mem as high as it will go stable.

What he said, should make your speed go up.

Here's mine for ~730k

cgminer -g2 --scrypt -I13 -w 256 --thread-concurrency 8192 --gpu-engine 1080 --gpu-memclock 1500 --gpu-powertune 1 --temp-target 80 --auto-fan

A friend of mine has an identical card to me but had to use an engine speed of around 1078 to get reasonable results.  I would recommend trying these and move the gpu-engine speed around a bit in very small increments until you find the sweet spot.

read this https://bitcointalk.org/index.php?topic=395199.0

I have 3, they all like their memory at 1500MHz any more or less hurts my hash but they all have different favourite core clocks, 1040, 1050, 1080 and they were different again on a different system. Play with your core clock from 1020-1100MHz in steps of 10MHz then fine tune from there. I also find when using windows turning off 'aero' (or whatever its called these days - fancy GUI bits) can help a little and setting the cgminer process priority to high helps prevent hash dips when fiddling with other things.

Here are my cgminer config settings, clocks handled by MSI afterburner;
"intensity" : "13,13,13",
"vectors" : "1,1,1",
"worksize" : "256,256,256",
"kernel" : "scrypt,scrypt,scrypt",
"lookup-gap" : "2,2,2",
"thread-concurrency" : "8192,8192,8192",
"shaders" : "2048,2048,2048",
"gpu-engine" : "0,0,0",
"gpu-fan" : "0,0,0",
"gpu-memclock" : "0,0,0",
"gpu-memdiff" : "0,0,0",
"gpu-powertune" : "20,20,20",
"gpu-vddc" : "1.100,1.100,1.100",
"temp-cutoff" : "95,95,95",
"temp-overheat" : "85,85,85",
"temp-target" : "75,75,75",
"api-mcast-port" : "4028",
"api-port" : "4028",
"expiry" : "120",
"failover-only" : true,
"gpu-dyninterval" : "7",
"gpu-platform" : "0",
"gpu-threads" : "2",
"hotplug" : "5",
"log" : "5",
"no-pool-disable" : true,
"queue" : "0",
"scan-time" : "60",
"scrypt" : true,
"temp-hysteresis" : "3",
"shares" : "0",
"kernel-path" : "/usr/local/bin"

I use this as a batch file for convenient booting, 60 sec start delay with press any key to skip, high priority process and maximised window;
@echo off

timeout /t 60
cd "c:\miner"
start /high /max cgminer.exe