I am running an 89 Unleashed tune. I just installed it last week.
I haven't done anything beyond installing fresh plugs at a .030" gap. The plugs I removed (at <27k miles) were at .034 to .035 (pretty good!)
I also have 87 and 93 tune files, but haven't evaluated them yet. I will likely try the 93 at some point but wanted to see if 89 will be 'good enough'.
So far, shifting is noticeably firmer (but it is adjustable softer/firmer at SCT tune load).
In sport mode, shifting is probably too harsh (for me). Again, that's adjustable, but I don't use Sport Mode much....so far
Power increase on the 89 tune is noticeable, but I wouldn't call it significant. Perhaps the 93 will provide more 'wow'?
Even so, the tune still adds a nice bump.
For example, a WOT pass from 65 to 75 is more 'daily driver acceptable' to me now. At stock, I felt highway passing was 'meh' at stock. Now its 'ehh, ok'.
My other vehicle has a Turbo 6 engine. I've run that tuned for a number of years now. It was a more 'night and day' result after tuning. But it also runs 93 and isn't my DD.
If I stay at 89 on the MKZ, I think I'll be content overall. I rarely go WOT and am a conservative driver 99% of the time. I do want max power for the right occasions.
Its probably premature to guess the impact on Fuel Economy. I'm pretty comfortable saying that FE has, at worst, stayed the same. At most optimistic, I may have gained 1.0 to 1.5 mpg.
I do worry a little on possible warranty effect, but I've worried a little on the 4 or so cars I've tuned under warranty. Tuning brings a little risk. Don't gamble what you can't afford to lose.
Anyway, I'll try to remember to update here as I collect more miles on the tune.