I´ve heard of very few people trying to upgrade the 4.0 ignition system. Those who have tried, mostly said it wasn´t worth the effort.
http://www.jeep4.0performance.4mg.com/ has some good pointers for increasing horsepower.
The distributor cap is a standard Ford, so I´d guess a larger cap (radius) could be installed to help prevent cross fire. But, all in all, the 4.0 ignition system, isn´t one of the weak points of the XJ. If they crossfire, it´s usually from some kind of moisture or conducting oxidation/dirt, inside the distributor and /or cap.
I´ve found a good set of ignition cables (OEM or high quality supression wires), Champion or NGK plugs, yearly distributor cap changes and some sealing of the distributor if fording, are really the only necessary maintenance and/or improvements necessary.
The distributor caps (Ford) have a tendancy to gather a coating (inside)of oxidized something, that can cause problems, that a larger distributor cap, might slow down but probably not cure. The stock igniton, is a high output, anything higher or improved, would not probably improve anything and would probably just burn up plugs faster.
Just a thought, the system does seem to be sensitive to resistance values, after the coil. Especially the coil to distributor wire. This wire requires a near stock resitance for the system to work optimumly. Don´t really know how a larger cap and the posible change in resistance, might affect the computer and ignition modules.
In an older distributor, the spacer between the drive gear and the base of the shaft shroud, is worth a close look for wear. Also the drive gears for the distributor, are probably worn, causing some slop. If the shaft is tight and the cam position sensor, inside the distributor is OK, it´s probably good enough.
The magnetic trigger in the distributor really has little to do with ignition, it`s just an indexer for the computer. The CPS (crank position sensor) is the main trigger for ignition.
