The two greatest areas of improvement, besides power increases, are aerodynamics and weight.
The XJ is a brick, with lots of frontal area and drag (just try to not add more drag if possible).
Lightweight is an XJ benefit, due to the unit-frame and modern construction. When considering other improvements (sound system, interior changes, accessories, etc.) make an effort to recognize that the added weight may impact the performance.
If in doubt about the weight, make it removable for runs at the track.
Remember flywheel weight has a significant impact on performance. Tall and heavy tire/rims will restrict the vehicle performance. The balance between traction and tire size should always be considered.