We run a similar FOX Thunderbird, while they still last I can recommend the Dunlop Direzza Z-II Star Spec. No news yet on the Z-III but if the roll out of the Z-II was any indication, expect high tread wear until the "Star Spec" version comes out.
Unfortunately in the Z-II the widest you can get in 15's is 205. the car came with 225/60-15's
225/45 gives you some options Goodrich, Khumo, Toyo
https://www.tirerack.com/tires/TireSear … iameter=17
Sadly the now "Official Tire of Lemons" doesn't come in 15's
When researching the car I was working on before the T-Bird the opinion was to leave the whole hatch with window on as it helps aero, keeps weight on the back, and keeps the exhaust from rolling back in on you.
Not sure with the 2.3, but assume maybe 100 on a really really good straightaway entry? MAYBE? Probably lower, and your speedo won't read right anyways with the 45 series tires, FOX Mustangs are pretty blocky cars and the NA 2.3 has a lot of work to do above 80 to shove the air out of the way.
Good luck removing the Asphalt Sound Deadener. It sucked. Dry Ice only Kinda worked on it.
Mistake By The Lake Racing (MBTL)
88 Thunderbird "THUNDERBIRDS ARE GO!", Ex Astris, Rubigo / Semper Fracti
A&D: 2014 Sebrings at Sebring (NSF), 2014 NJMP2 Jurassic Park (SpeedyCop), 2012 Summit Point J30 (PiNuts)
2018 Route Sucky-Suck Rally Miata, 2019 World Tour Of Texas 64 Newport