The exhast, head, intake and then gearing are the main areas.
At the very least, bring it up to ED XR6 spec, and keep it a low profiler!
First, look at the correct XR6 spec mods to the exhast. The people who flog off single pipes as a replacement to Factory XR6 or EF GT items should be ashamed. Big drainpipes don't do much to help. The best option is to reduce backpressure by changing to the EB to EL dual side by muffler set-up which keeps exhast speed up without creating an awfull resonance. The tail pipe could be brought up to the EL GT spec.
The head recieved compression, cam, valve guides and spring settings were blueprinted on the EB XR6, and the ED gained the better valves, and some basic mods.
An expensive but very valuable upgrade is the intake system form the EF. It is still being used on todays fly-by wire BA Mark II, and its very good. Although difficult, the swap of the EF,EL manifold and electronics as found on the Ghia or XR6 is a good move.
The use of the stout 3.45:1 gears, espeically with the harsher and harder LE 95 shift settings or the old T5 is a good change. The stock 3.27:1 gears were an improvement on the old 3.08's, but the 3.45:1's are best. The aftermarket Kyver 2800 torque converter is also a good option.
Later EF, EL, AU, and BA's are much heavier, and had constantly upgraded engine specs. 161, 164, 170 then 182killowatts. And how much faster? A non turbo XR6 182 kW is hard pressed to bust the 15.5 second quarter mile barrier with 1750 kilos while the 149 kw XD Falcon would do 15.8's all day with only 1450 kils to haul. We've all been duped! Despite twin cams, variable valve timing, electronic throttles the latest 2005 XR6 stocker is slower than the first 1992 edition. The earliest EB XR6 remains the fastest factory XR6 over the qaurter mile at 15.2 seconds
After all that, electronic mods eliminate the factory factor of safety Ford builds into every car. Chip mods are to be avoided unless you are prepared to get into the engine. They are the last option. I have seen cars like EB- EFXR8's see huge gains with aftermarket chips. Sixes seem to be pushing the envelope more and don't see such a huge gain.
$$$$ again...
