I know I'm reviving an old thread here but having just replaced my CAT600 bushes with the IMS kit, I thought I'd share my thoughts on both setups.
The CAT600 setup with Nolethane bush served my well in my Kenne Bell supercharged XR8 for about 6 years and 50,000 k's with daily driving and the occasional thrashing. There was increased NHV as was to be expected from such a stiff type bush, but it was not unacceptable.
Recently the CAT bushes had enough and thrashed out so I replaced them with an IMS diff hat kit with the Superpro bushes and I was very surprised how much quieter it was compared to the old bushes, even when they were new.
The car also now has zero axle tramp which it still did have a little bit with the CAT bushes and running Shockworx coilovers.
So the CAT bushes were good with the standard layout but the IMS kit, I think, addresses a basic design flaw in the original Ford design with out really increasing NVH. I'm very happy with it and recommend it.