Before trying anything else I would replace the front O2 sensor and do a good tune up to make sure that there are no drivability/ tune issues. O2 sensors only last 50-60k under ideal driving conditions. They get lazy and do not switch between rich and lean as fast as they should. I would not recommend a 4.0 TB without first upgrading the exhaust. While I have not performed this modification I have performed similar on other drivetrains. The theory of engines being fairly constant I would say that what increase or decrease that you get depends on your driving style. It will affect the power curve of the motor. At low RPM you may loose torque at high RPM you may gain horsepower. So my opinion would be that it could improve hi speed driving, but off road where you need torque it may decrease performance. So it would depend on the balance that you are looking for.
After about 200 miles of driving the fuel trim/ plugs need to be checked to ensure that it is not running too lean, which will cause major issues.
Best of luck. I learned a long time ago that what works for one person dose not work for everyone. That is why they call it tuning.