Its been a while since I built a Linux rig so I dont have any specific suggestions, but the 540KH/s rate definitely indicates it isnt clocking the card as high as it should be. As you are using the same config as you get 730KH/sec with elsewhere it suggests driver problem to me, especially if you arent getting GPU stats.
I have seen people say that they had problems with the 14.x Catalyst drivers, so if 14.4 doesnt work maybe try going back to 13.1
I have seen people say that they had problems with the 14.x Catalyst drivers, so if 14.4 doesnt work maybe try going back to 13.1
Thank you for the response. I have been busy recently, sorry for the late answer.
I reinstalled Ubuntu and 14.04 drivers. Cgminer and sgminer now shows GPU info, but the problem was not solved.
I explain the solution for anyone who has encountered this issue. The problem is the kernel of the cgminer and sgminer (scryptxxxxTahitixxxxxx.bin file). I had a bin file in the nice working cgminer which makes it produce more hashrates. There is another one here:
http://www.reddit.com/r/dogemining/comments/1uj4uk/optimized_scrypt_kernel_for_hd7970_r9_280x/
which is a nice one. There is an instruction how to use it in cgminer on the website. To use it in sgminer just rename it from "scrypt130511Tahitiglg2tc8192w256l8.bin" to "ckolivasTahitiglg2tc8192w256l8.bin", backup your older bin file , if there is any, delete it and copy the new one in the sgminer folder.