It depends on how the damper's and springs are set. Look at a BMW 3 Series, it uses IRS, but nobody would ever compare it floating in air. Of course an Explorer is going to feel like it floats on air, that's what it was designed for and given the market for it, the ride and the handling justify the damper and spring settings. In a Mustang, the IRS would be designed with a view towards handling "fun to drive" factor. The ride will improve, but it won't be a "boat" by any stretch of the imagination.