The T-5 IRS is touted as a bolt-in system, and it was originally designed that way. On the one modern installation of it that I've seen, the builder (a mechanical engineer) wasn't very comfortable with it and welded in a bunch of reinforcement.
From the research I've done into the system, Ford was developing it in part for use in the '65 G.T. 350. Shelby tested it and concluded that there was no real performance advantage to using it in the Mustang platform and he could get better handling out of the solid axle in his race cars. It was also expensive, so it was shelved.
In my opinion, when you start a design with the huge constraint of making it bolt in to an existing package, you've got one arm tied behind your back from the get-go. You've got to make engineering compromises you normally wouldn't make. The SVT guys faced the same problem when designing the modern Mustang Cobra IRS.