Bear in mind that's only 404 lb ft, it is not a monster engine in production form, and the car weighs >1700 kg. Other sites claim 5.7 seconds. To be honest until they sort the clutch problem out no-one will really know.
So far as the Mustang goes, for an IRS you'd be putting six hundred bucks and some kg into a car that sells for as little as 20000, for a benefit that few people would notice, and which certainly would not affect their buying decision.
I wish we'd kept a coil spring beam axle variant going in our car, it really was a sensible solution for real people.
Cheers
Greg Locock