Depends on what you mean by performance...
BBK makes a Throttle Body
iFab makes a polished 'intake' (not REALLY an intake, but that's what people call it)
ASP makes Underdrive Pulleys for the 4.0L OHV (same as the SOHC)
There are about 3 manufacturers of 'chips' for your X.
There are a couple different manufacturers of Exhaust Headers for you as well.
One of the most 'modded' Explorers around, is Al Franklin's. He goes by aldive on here. A COMPLETE list of his mods are here:
http://www.explorerforum.com/forums/showthread.php?t=137531
He has a 4.0L SOHC, you have a 4.0L OHV, not ALL of his mods are available for you. But you can see what CAN be done. I'll also add he does his mods PURELY for increased mileage. But a major factor to getting better mileage, is you have to make the engine more efficient. When you make an engine more efficient, it can make more power.
Ryan