Let me start by saying that my 5.0 is completely STOCK except for the removal of the rear two cats.
All FoMoCo except the coils are MSD, hey I like the red.
The trans is as Ford built it except for the 4406 t-case.
It has the approximate 2" t-bar and 3 1-2 - 4" rear lifts with 33x10 1/2 tires, and still has the 3.73 gears.
On my x4 there are 3 box tunes for 3 different octane rated fuels, and you can make adjustments to many of the settings like, idle, spark advance, fan temp on/off, etc.
I run the regular octane tune with a few tweaks and, yes there is a noticeable difference.
Mine runs cooler, feels like it comes on smoother when opening it up, and goes a little further on a tank when travelling on the highway.
If I put some premium in and run the premium tune, it wakes up noticeably.
But without premium it'll detonate like crazy with that tune.
I haven't tried the mid grade tune.
On the phone once, an SCT rep told me that the x4 tuner for the Ford wouldn't work on the Mercury and that I needed to buy the x4 for the Mercury which did not come with any box tunes and buy a couple of custom tunes.
My Ford x4 has worked great on my 2000 Mercury.
The x4 when married scans the vin and mine comes back as 2000 Ford Explorer (?).
I really don't understand what he was talking about.