I don't agree with Opie. with the high flows and no cats. Itcomes down to the size of the exhaust piping that make the difference.
We also tested a few high-flow cats at the time and saw that those, depending on brand/style, provided minimal HP gains yet still reduced mileage all with acceptable emission levels........these claims via "butt-dyno" testing, not and repeatable or consistent testing method such as a dyno.
Well my mileage never changed. I was still running about 29-31 mpg on the highway and I didn't need to use my butt dyno. With the highflow, I went from a 15.5 1/4 mile to a 15.1 with just the high flow. Top end on the car was much better. The only draw back is the CEL coming on once in a while for Catalyst System Efficiency Below Threshold and the CEL fix would get rid of that. I might get a chance to test both the highflow and no cat at a dyno later this year.