I had an R/T Hi-flow cat & didn't have any issues with it. Doubt they still make'em though. A SVT Header will fit any Focus with 2.0 Zetec which were only made from 2000-04.
"I flushed a bag of skittles down the toilet yesterday and watched a NASCAR race in 8 seconds flat."
06 Spring Into Focus Best Sedan/Wagon Winner
03 SE SVT Wannabe (sold) to help pay for my 08 Cooper Red Mazda CX7 AWD GT