I've had similar problems (but with different cards in on PC).
Try set parameters directly for each GPU:
-I 13,13,13,13 -w 256 --thread-concurrency 8192,8192,8192,8192 -g 2 --gpu-engine 1040,1040,1040,1040 --gpu-memclock 1500,1500,1500,1500

and, after launch, check with GPU-Z or MSI AfterBurner gpu voltage and memclock for each videocard.
For me, cgminer often ignores frequency set by commandline so i had to set it directly from afterburner.

For my XFX R280x black, those settings do 680-740 kH/s: --scrypt -I 13 -g 2 --lookup-gap 2 --thread-concurrency 8192 -w 512 --gpu-powertune -20 --gpu-engine=1030 --gpu-memclock=1500 --no-submit-stale

Forgot to add: 750w PSU probable not enough. 650w is minimum to power motherboard/hdd+ one 280x - when I used 500w psu for some time, my pc reboots several times a day.

Sorry people, stupid me, asked before dig slightly deeper
I've found the solution: synchronize --gpu-vddc, --gpu-engine and --gpu-memclock  - and everything OK!

SET OPT=--gpu-platform 0 -d 0 --gpu-fan 95  --scrypt -I 13,13,13 -g 1 --lookup-gap 2 --gpu-vddc 0,1.080,1.080 --thread-concurrency 8192,8192,8192 --gpu-engine 1100,980,980 --gpu-memclock=1650,1400,1400 --no-submit-stale

SET OPT=--gpu-platform 0 -d 1,2 --gpu-fan 90 --temp-cutoff 76   --scrypt -I 13,13,13 -g 2 --lookup-gap 2 --gpu-vddc 0,1.080,1.080 --thread-concurrency 8192,8192,8192 --gpu-engine 1100,980,980 --gpu-memclock=1650,1400,1400 --no-submit-stale

Hi!

Added 7970 to my 7990, and *shocked*. Cards in separate PCs gives 1300+510 KH/s. Combined together in one PC, they gives me fantastic 960+540. No matter if I run them altougether with -g1 or -g2 options, or launch from different cgminers (one with -g2 for 7990, one with -g1 for 7970).

I've even lauch cgminer on separate CPU cores, but no effect.. Once I run 2nd instance of cgminer (from dirfferent folder), 7990 lost over 300 kH/s. 7970 feels OK.

Can anyone experienced point me to my errors?

commandline opts are:
SET HEADER="7970"
SET IDS= XXXX
SET OPT=--gpu-platform 0 -d 0 --gpu-fan 95 --scrypt -I 13 -g 1 --thread-concurrency 8192  --gpu-engine 1100 --gpu-memclock=1650 --no-submit-stale
start /AFFINITY e  cgminer.exe %OPT% %IDS%

SET HEADER="7990"
SET IDS= ZZZZZ
SET OPT=--gpu-platform 0 -d 1,2 --gpu-fan 90 --temp-cutoff 76   --scrypt -I 13,13,13,13 -g 2 --lookup-gap 2 --gpu-vddc 1.080,1.080,1.080,1.080 --thread-concurrency 8192,8192,8192,8192   --gpu-engine 980,980,980,980 --gpu-memclock=1400,1400,1400,1400 --no-submit-stale
start /AFFINITY 3  cgminer.exe %OPT% %IDS%

Hi!

XFX  FX-799A-6NF9 gives 1.3Mh/s, with core voltage 1100 mV, 985 Mhz GPU freq, w/o powertune.
cgminer options is -I 13 -g 2 --thread-concurrency 8192 -w 256

On my opinion, two R9 280x (my R9-280X-TDBD gives 710 kH/s each) or R9 290 (even more kH/s) will cost less then one 7990 and bring more hashrate.

Hope this helps.