It's only objectionable between 1100 and 1500 under light acceleration or climbing a hill in too high a gear. It's only a 2 or 3, so I'm living with it. At least for now, it's not a problem.
I have the MF Comp axle back with 3.73 gears. I like the deep, smooth tone but I have some drone between 1100 and 1500 rpm and a little less drone from 1500 to 1900. It's not bad, but it makes me be aware of what rpm/gear I'm using when driving easy in my small town.
Would be interested in seeing your results. Afe is way less expensive than Kooks and if the cats don't throw codes, they would be a good option in my situation, too.
Damn, I was estimating 2" ID. It's worse than I guessed. Have you seen any explanation from Ford as to why they added the flex pipe? Planned on keeping the factory manifolds / cats, but that neck down has to go!
Filled up nicely with steel wool or packed glass or some similar material? Its difficult to tell from just seeing what appears to be shiny strands of something inside a few of the louvers