I like H-pipes in dual exhaust systems for two reasons.
It will attenuate the peak spike pressure from any single exhaust event, effectively reducing average backpressure.
It makes the exhaust noise "mellower".
Put it as far forward as practical. Most are exhaust pipe diameter, but a smaller one will work also.
I've never run one on a six cylinder on a dyno, but on lots of V8 dyno tests I've run we picked up on the avreage of 5hp (245 HP engine) with an H-pipe.
