I actually tested circle hooks for tuna last year and I was surprised the hookup ratio on circle hooks on jigs were as good as J hook.
The Canyon Runner, one of the prestigious charter boat on the East Coast has used circle hooks for tuna to release tuna for years and they report circle hooks for jigs are fine.
There was an article in one of the SW mags about this. They determined a slight offset circle was best with a jig. I cannot remember, but I think he was using in place of a treble; not on an assist hook.