I ran mine without cats for quite a while with no problems. Some have said there is a loss of low end power on the automatics but I've got a manual transmission and it felt like it had a little more power to me.

2001 Sportage, TJ 106's, Everet spacers and bumpstops, Bilstein shocks on the rear, cone filter, Crager wheels, 235-75-15 Goodyear tires. Custom front winch bumper by Dave Scott. Currently DOA, but it is a work in progress.
2011 Kia Forte EX