Backpressure is never good for any engine. You want to eliminate backpressure and maintain velocity, hence certain size of pipes are appropriate for particular engine builds. I'm under the impression that with a turbo it matters much less after the turbo, you just want to avoid restrictions that cause backpressure. Before the turbo velocity should be maintained with appropriate size pipe, smooth bends, and coating or wrap to retain heat (volume, velocity).
If 3" after the turbo is adequate, then my guess is 6" won't hurt it (but also won't help). What I don't understand is why many guys go from a 3" DP to a 5" or larger exhaust with even larger tip. I could see maybe 3" mandrel bent DP to a 3.5" regular bend pipe since the diameter will be a bit smaller on bends, but what does 5" or larger gain? Just seems like it would be a pain to work with and take up a bunch of unnecessary space I'd rather use for other things. Maybe there's a logical reason for it, but my supposition is that it's the diesel equivalent of fart cans.
