Don't know unless you disclose how much voltage you're using. No voltage increase, yeah it's safe. I've been running my cards at 1000/1250 for the past week with 1.225v going to the GPU. No signs of fire yet.
From my experience you are way beyond the point of 'purposeful returns' in terms of the performance that you will get. I copied your clock, but as I was working up towards it, I noticed strong diminishing returns and much higher temps occur around the 950MHz on the GPU mark.
On Heaven (Maxed out 0FSAA 4AA DX11 Tesselation Enabled):
@ (stock) 1.087v, 725/800 = 33.9/853 points
@ 1.137v, 900/1135 = 39.4FPS/993 points
So a very worthwhile overclock here with the GPU sitting at 69C under load (fan speeds set to AUTO)
@ 1.16v, 950/1200 = 40.7FPS/1025 points
The above overclock however sends the GPU temp way up to 80C under load (fan speeds set to AUTO). Although I could have altered the fan speeds in order to safely maintain this clock and indeed clock it even higher, it seems to me that the end product (i.e. actual increase in performance) just is not worth all the extra strain being put on the equipment.