Micheal Yount is 100% correct.
Anybody who asks if Ford will void a warranty as a result of a tune has no knowledge of the 6.0 Powerstroke. Born as the International VT365, it was factory hot rodded for the pick up truck HP wars. No international truck commercial truck had a VT365 producing as much power as a 6.0 Powerstroke. Add iffy emissions controls, crapping high pressure fuel pumps and idiot tuners cranking in an extra 200 HP (diesels are ridiculously easy to "tune," just add fuel) and things were blowing up everywhere.
Ford sued International. International almost went bankrupt. FORD KNOWS TUNERS.
I always laugh when the naive quote the MM Act. Walk into any car dealer with a warranty claim and the dealer must follow a strict diagnostic procedure. If there is any sign of modifications and specifically tuning (Ford has diagnostic tools that can find a "tuner footprint" even if the original tuner has been reloaded). The dealer MUST report findings to Ford. you have to ask yourself what dealer in his right mind would jeopardize his franchise agreement by lying or omitting information on your warranty claim?
As far aftermarket parts manufacturers guaranteeing warranty compliance, that's even more laughs. Read the "guarantee."
Many moron Powerstroke owners ditched their OEM intakes and air-filters for K&N style units. K&N makes some sort of promise about not voiding warranties. The fact is Ford supplied Powerstrokes with Donaldson air filters. Donaldson air filters flow better, filter better and can hold pounds of dirt without loss of performance. Google "dusted Powerstroke" to get an idea of how many guys ruined their engines by running a washable filter off road. There are hundreds of warrantee denial stories on the internet. i have never found one story of any washable filter maker paying to replace any engine after a manufacturer warranty turn down.
I don't know about the latest Mustangs, but Ford diesel pickups have large decals in the engine bay specifically prohibiting modifications.
Look what I found. a genuine Ford Technical Service Bulletin about aftermarket mods on Mustangs - specifically tuners - complete with that diagnostic chart I mentioned. Of course it's followed by Mustang Daily's editorial comment about the MM Act.
Ford issues TSB warning against modifications and aftermarket parts for the 2011+ Mustang | Mustangs Daily
P.S.: Canned tunes are BS. Put your car on a dyno or don't bother.