I would advise that larger bars can have clearance issues with the headers, I don't think you will have any issues if you stay under 1.10" but above that they can be tight.
If you are buying used bars watch out for the clocking of the hex heads as they are all over the map and some can be a pain to get the correct ride height with.
I would also replace the ride height adjuster bolts at the same time as you install stiffer bars as they may be more prone to stripping with stiffer suspension.
If you really start lowering the car with a factory front end you start running out of suspension travel, on bad bumps it will bottom hard on the bump stops.
I would say if you are not racing the car go with the PST 1.03 bars or maybe Firm Feel 1.06s, they should give you about the new Mustang ride quality that gets tossed about.
