[David Pozzi]

A Ford 9" with no floater and 14" rotors, 6 piston calipers weighs 195 lbs.
A Ford 9" with floater and 14" rotors and 6 piston calipers weighs 220 lbs.

To compare to the Morrison, you would need to know the weight of the shock mount bar, panhard bar, brackets, and 3 link equipment.
According to Morrison, their IRS is 30-40 lbs heavier total than a Ford 9" with suspension, but it has an even larger ring gear & is stronger. The added weight is low and to the rear which is the best place to add it if you have to add weight. The un-sprung weight drops from 150 lbs per wheel to 80 lbs assuming a 50 lb wheel and tire.

In terms of ride, IRS wins hands down, it's way way smoother riding & you will enjoy driving the car so much more on the street, it isn't funny.

In terms of autocross times, IRS will beat a stick axle with equal tuning time invested. How much? I don't know if it's a second faster, but somewhere close to that.
How much better on a road course? I don't know, but if it's a bumpy track the IRS will be faster, since lap times will be longer on a track, I think easily a second faster, maybe more, but I'm guessing. You will feel more confident going over rough sections of the track.

For a car to drag race, the stick axle probably has an edge in theory, more anti-squat could be used, but there are plenty of late model Corvettes doing pretty well at the drags.