I have the  same issues as  mrestey and I have a similar hardware set up....  I have tried all you mentioned    setting shading rate  to  >1 , bucket size to 8  and all that  the it gives me is  a little
farther down in the window before the memory error sets in.
The scene  is quite dense and complex  in content. It renders without error in P7  and PP with
Firefly. This leads me to believe that P8 must be using more memory to accomplish the same task... Are there any caches or such  that might need clearing that might help with this situation?
I hate haviing to keep PP  on my computer just for big scenes.