IRS is gonna be the only
suspension mod that will make your ride quality nice on the street. Even from the factory, the solid axle stangs dont ride well over bumps, but the 6" of wheel gap, & soft dampers make the ride more comfy. IRS rides like a dream, even when lowered. My buddies 03 Cobra lowered on sportlines rides so smooth, its hard to tell your riding in a mustang....
I have the full MM front
suspension & most of the rear
suspension on my Mach, & although the car handles great, it rides like complete crap on the street. Its so uncomfortable, I cant take it on long trips. Some of that has to do with the fact that my car is dumped, but the solid axle is the main reason combined with the super low stance & lack of
suspension travel.
I have considered doing the IRS swap because I love the ride quality & handling benefits, but IMO the downside is the amount of weight it adds & the severe wheel hop with the stock IRS parts (IE: bushings etc).. The weight gain isnt much of a burden on a blown car with 400+ rwhp, but on a N/A car with only 230-330rwhp, it makes a huge difference IMO. BTW, the IRS weighs 75-100 lbs more than a solid axle. Launching an IRS car at the strip isnt that great either & the wheel hop gets pretty bad unless u do the complete bushing kit, & the braces ETC so that is something to consider also if you like to drag your car...