An X-pipe full time merges the exhaust pulses from each cyl bank (even when high pressure pulses from opposite cyl banks reach the X-merge at the same moment, they are forced together creating a pressure spike) this will create rasp. An H-pipe on the other hand, will ONLY allow exhaust gasses to pass to the other pipe when there is a "between exhaust pulses" low pressure zone in that pipe, this will be louder and give an increase in torque.

There is significant data posted online by those who have taken the time to dyno exhaust systems. On V8 powered vehicles, X-pipes with UNEQUAL length tubing from the cyl banks leading into them (as on V8 E53s), DECREASED torque, when H-pipes were installed torque increased. Henessey Motorsport is an example, when developing the Hemi SRT Jeep package the torque figures took a dip when an X-pipe was introduced into the exhaust system.

As a bonus, the H-pipe will increase the low frequency rumble whereas the X-pipe increases the raspy higher freq. It all comes down to what you want louder.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!