You have to define better.
Does your XJ, pull a trailer better with a slightly restricted exhaust at 55 MPH, likely.
Does it accelerate through peak torque quicker on a flat street into the upper RPM band and maybe loose some torque in the process with a more open exhaust, likely.
horsepower = torque * RPM divided by 5252
You can lower one and raise the other, move them around in the RPM band some and mess with peak RPM. But in the end you have three variables and one constant. Messing with the exhaust changes the curve some.
Most XJ's are heavy, have a motor designed more for torque than peak horsepower, often turn big tires and have mechanical limitations on the highest RPM's that the motor will survive.
I do a little street racing from time to time. I shift early and try to keep the motor in the peak torque band. I regularly beat lighter cars with a better horsepower to weight ratio. How? Because my peak torque band is flatter than their peak horsepower band. My steady acceleration can beat there peaky motor and quick shifts. If I drive it much out of the the torque curve, which falls off fast after mid RPM, they win. I also try to pick my road, and try to stage the race from 15-20 MPH through around 75 MPH (lower if possible). HO motors have moved peak torque a little higher into the RPM band and seem to have narrowed the torque curve a little. Butt dyno.
My HO is marginally a little faster than my Renix, but really not much. They accelerate a little differently, but the end results are largely the same.
I imagine there is some room for tuning the exhaust. But it would likely be easier IMO, to tune it out of the envelope, than to get it much better.