Which brand? You need this info to make an informed decision. If it ends up being a 9.5" for the front I wouldn't go above a 275 tire. A 285 will fit but won't be optimal, especially for the front.
The PP on the Mustang is the last benchmark I would look at. The PP cars are so undertired. You have a 3900lb car with a 56/44 weight distribution and only a 9" wide front wheel with only a 255 tire.
To the OP, rethink your wheel choice I wouldn't go below a 10"wide wheel for the front...
That is a perfect Mustang fitment. I ran Mich PS4S up front and the Mich PSS out back (no PS4S in 305/35/19). My S550 with this setup..I will be running the same setup when my S650 arrives.
The only issue, if you want to go with the DH 10.5 front you will have to go custom increasing the cost exponentially. I would probably just live with a 10" wide wheel with a 295/30/19 up front and a 315/30/19 out back.