[aerohead]

Engines are like Algebra.Whatever yo do to one side of the equation,you've got to do the the other.
An exhaust mod won't see top value without mods to the induction system and vice-versa.And new camshafts are usually involved.
Relieving back- pressure for the rpm and flow range in which you'll typically be driving,might, and probably would exacerbate your situation.It's a challenge to beat powertrain engineers at their own game.
You may just want to respect the limitations of the car and enjoy it's good points when the driving environment allows.
A large-displacement engine with flat torque curve would allow you to walk up grades without a downshift,but then you'd enjoy all the dubious benefits of a large gas-guzzling engine.
You might just stay in the slow lane,let your speed fall,and let other traffic pass.