The benefit of a lower tire pressure in our land yacht Town Cars is a softer ride, regardless of city versus highway. It absorbs the impact of potholes, bumps, etc. much better, which is easier on passengers, that you may want to impress, and the vehicle to some degree. But, 28 psi with 4 American (big) adults in the car isn't such a good idea, as it may distort the sidewall too much.
In my opinion the std. Town Car air ride suspension, and even other Panthers with coil springs, has enough cush built in to it to provide a sufficiently soft ride, so I stick to tire pressures of 32 to 34 for all conditions. Which is what Ford or any tire manufacturer will tell you.
KBX