In my opinion you can chase all the gremlins you feel may be causing your chine walking but in the end there is such a thing as exceeding the maximum speed the hull was designed for and I think you are probably already there. Upgraded steer may help you control the chinewalking better but it would seem to me that riding lower in the water with the same HP = slower. I could be wrong.

Hulls do have limitations. 70 is not this hulls limit.
This hull can obtain 70 mph speeds without being dangerous.
If the stern lift is decreased the boat will become more solid. Yes this can create more drag than out of control chine walking but, if the boat is managable, the ability to add more trim and decrease the wetted surface area up front will out weigh the stern drag and you will go faster in control.

Brett