Started playing around with the mixing a little. The suggested value of 51 got rid of the oscillation that I was having in sharp banks, but still would have the rear of the quad drop while doing yaw. Out of curiousity I bumped up motors 3 and 4 to a value of 70 and the oscilations were severe.
I then tried it at 60 and they were better and yaw was normal. Dropping down to 55 almost gets rid of the oscilations completely and the yaw is still pretty dead on. Will try 54 next.
I guess given the design of the hellbender I can't tune the quad simply with P & I tuning, but its getting pretty close with the custom of 64 64 54 on the respective motors.