I have caught everyone of my sharks on circle hooks. For 7's I use 13/0 hooks that are about the size of 16/0 mustads used very successfully by the east coaster sharkers. I do open the gap about 1/8 - 1/4 in. On sharks with big teeth, if it doesn't set directly in the corner, it's probably getting caught between the teeth and popping loose without actually catching skin. On the east coast, when they are hunting big sharks with big teeth, the hook of choice are 20/0 which are big enough to get around the jaw bone as well if it doesn't catch the corner. I believe I am somewhere in the 90% bite to hook and land ratio with the hooks I use. Plus at 5.99 per 25 hooks, they don't hurt the wallet. Last year, most of my sharks were also on hooks with the barb pinched down. Once a circle truly sets, it's almost impossible for it to come unbuttoned unless it tears loose.
I agree with circles, it's very important how you hook the bait to allow clearance. I always use bait thread to lock my bait in the position I want so it can't rotate and get the point buried back in the bait. I have been meaning to get some picks of how I rig my bait. Maybe I'll get some up tonight.