The one with the IRS was fitted it to try and find more traction off the line strangly enough, a new mod a cortina builder in essex is trying out (Steve Taylor)only seen em on 24vs so far.
We just did a CF shoot at brunters with the IRS car getting off better and recording a quicker 0 to 60, that was a 24v Mk3 with IRS and LSD againt a 2.8i turbo Mk5 with usuall rear end, mind you the Mk5 didn't have a slipper fitted which deffo didn't help

Check out next months CF ,it's a cortina special will have the results/artical in it
I must say a sierra rear end deffo feels more predictable in the twisty bits with a corty rear end (specially when it's fully polybushed) it can be a bit "unpredictable"