I don't think the HP/TQ improvements from full length low restriction exhaust systems are a value. Very costly based on what they deliver. If there is gain from H or X pipes info I have read says it will be greater with the X pipe. Unless it is brand name full length system I would go with what delivers the sound you like. H and X pipes sound quite different. Might want to make the changes one at a time. I would also try dropping the muffler section to see what you get before doing any welding. I think the muffler is more restrictive than the resonator. There is a long thread about member results from X and H pipes that you can pull up in Search. Another options are electric cutouts. I have them installed just after the front cats. Can be stock to open exhaust or anything in between at a touch of a button.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas