So, I have some mods on my agenda, just curious what is proven to provide a better result. Also, how about replacing the cats with high flow ones?

Oh, the tough decisions we have to make in life.

Ideally the X or H should be placed at the point after the collectors or CATs where the exhaust cools. This is determined by using temp paint or tape that is 50-100 degrees below the Cat or collector exit temp.


For any performance exhaust system, some type of crossover connecting the two sides of a dual exhaust system is important because it acts to balance the two banks of the engine. The common H-style crossover is good at balancing sound pulses between the two halves, but does little to promote scavenging because the exhaust gases tend to follow the path of least resistance, which is straight through each pipe rather than taking the 90-degree turn through the H-pipe into the other half of the system. In an X-pipe system, however, where the two sides of the system intersect, the gasses have no choice but to intermingle as they pass through the junction. This promotes improved scavenging effects by smoothing out uneven exhaust pulses from the engine’s firing order. It also helps quiet down the exhaust, resulting in a mellower, less raspy tone. According to Magnaflow, the faster acceleration of the gasses through an X-pipe causes them to flow in a linear fashion parallel to the walls of the tubing rather than tumbling. This “laminar” flowing gas is much quieter than tumbling gas, resulting in an exhaust tone up to 8 decibels quieter than a traditional H-pipe.


X-pipe intermediates, according to certain dyno tests, seem to aid torque. Noise changes are a by-product, as the exhaust pulses are altered over those of an H-pipe system. As "power" increases, so does the X-pipe's benefits. Many NASCAR (sorry... Nextell) CupCars are now using variations of X- and 180-design systems to enhance torque and/or finetune power curves.


Swapping out the H-Pipe for a real X-Pipe does a few things. First, it eliminates the structural feature that causes the bell-like jingle common to the H-Pipe. Second, it benefits exhaust flow since the crossover is now the full diameter of the exhaust pipe. Exhaust pulses now split fairly evenly when they hit the crossover, making the after-crossover exhaust diameter effectively double the diameter of the OEM setup.


In addition to all this, an X-Pipe provides benefits in reducing backfiring. X-Pipes also alter the exhaust tone (eliminating some of the "bark") and generally quiet the exaust down.


Flowmaster.com:
Is an "X" pipe better than and "H" pipe?
Over the years, Flowmaster's testing of all types of crossovers, including "X pipes", has revealed no substantial benefits over a properly installed "H" pipe on street driven applications. However, in race applications with small cubic inch engines and /or restricted (small bore) carburetors, will respond well to the addition of X pipes, tri-y styled collectors and/or properly designed single exhaust systems.