I am having this problem right now as well.
I know it is my cats for the simple reason that I get that rotten egg smell all the time, sluggish acceleration, and when I am on the interstate at 1 and 2 in the morning, I sometimes let her rip to clean out the engine and get her up to about 120. It takes so long that I feel as if the car isn't an SVT anymore. I could easily go past 120 when I first got her. Now, it takes a long time and it will never go past that mark. My engine is at a steady 5K RPM level and even at WOT, the car just doesn't go anymore. So I am going to get my cats cut out and a straight pipe welded in.
Just letting you know that I can relate. Get those cats removed or replaced.
-Bishop
