The OP just wants to "restore its ride height, and give it some firmer handling".
I don't want to get in a pissing match over wheel size but these are some things to think about...
The less side wall, the harsher the ride and you do start to reach a point on larger wheels (like when going to 20" or another crazy size) where the profile of the tire is so short that the contact patch shrinks and becomes very thin...
Another thing is wheel weight.... Many fall into the trap of buying by size, with no regard to wheel weight. Big, heavy wheels will make your car slower and corner worse.
Oddly enough... Factory options (like the Mustang wheels)are usually lighter than normal consumer after-market wheels.
So.... If you do go big just make sure you shop around (take a look at wheel weights) and make an informed decision.
