I know it's been asked on here several times about HP gains from removing the Cats. The CT boys have done it and found no improvement in performance but they have found quite a bit of gain by using High Perfomance Cats like the DynaTech PowerCAT.

http://www.afabcorp.com/AFCO_Dynatech_USbrake/merchant.mvc?Screen=CTGY&Store_Code=Dynatech&Category_Code=Cat-conv

CT Thread: http://www.clubtitan.org/forums/showthread.php?t=38254

Key Search Words: Aftermarket High Perf Flow Performance Cat Cats Catalytic Converter Catalyst

Here's an example of some problems that can be caused by gutting them.

http://www.clubtitan.org/forums/showthread.php?t=38456