A couple of you have stated that that backpressure helps low end performance; this is not true. It is the increased velocity of the exhaust gasses with reduced diameter pipes that causes the improvement, not backpressure. An X crossover should actually decrease backpressure on tuned length headers on an inline six, as the exhaust pulses will be interleaved and flow through two pipes rather than one, as Joe pointed out earlier. If backpressure helpd anything, you could put a restriction anywhere on the exhaust system, including at the end of the tailpipe, to acomplish that goal; we all know that sticking a potato up your exhaust pipe will not improve performance, but it certainly adds backpressure.
