How much can I overclock this r9 280x?

having trouble following your links, they don't seem to work for me, but I can get my r9 280 with a sapphire dual-x cooler (a 280 non x version) to 1,080 mhz without increasing the power draw at all. If you just look up r9 280x overclock, you can see that most people get a bit over 1100mhz. Some of these pages...

http://www.anandtech.com/show/7400/the-radeon-r9-280x-review-feat-asus-xfx/21

http://www.hardocp.com/article/2013/10/14/asus_r9_280x_directcu_ii_top_overclocking_review/#.VZIW3vlVggs