Just a thought..................if an "X" pipe cross-over HURT HP,why do most people GAIN HP when going to a MagnaFlow from stock ? ( in case you don't know,the MagnaFlow has an internal X)
If you look at it,the stock muffler is wide open and straight through.
I think one of the reasons the MagnaFlow sounds so good is due to that internal crossover.
I ran mine with no cats and stock muffler and it sounds horrible.With just an "H" pipe (simplest type of crossover) it sounds MUCH better.
The only way that I know we can find out for sure would be to dyno test each combination.Otherwise we are all just speculating and guessing.
One FACT that I can put out there.............for whatever reason, the crossover DEFINITELY improves the sound (stock muffler,no cats)!!!!!!!!! Whether or not it would provide any additional improvement to a MagnaFlow equipped truck or not I can only guess.My guess is it will also help the sound on these systems.
For me,the sound quality was more important than a tiny bit of horsepower lost (IF there is any lost)