Any backpressure isn't good for an engine. But remember that an exhaust setup is only as good as the other modifications before it. The intake, throttle body, intake manifold, heads and cams all dictate where the power will be made. I thought about doing a 3" true-dual setup on the Titan. But without any other options for an intake manifold/throttle-body the VK56DE is strictly a low-end/mid-range motor. It's not made for high-end performance where a true-dual or any extreme flow system would shine. I also have speculations that the VK56DEK (07+ up motor) might have more then just IVTC in comparison to the earlier VK. Why stick an entirely different cam control system and intake manifold on a VK without opening up the head up a bit?
I'll be finding that out soon enough.
So for the VK56DE, I think a 2.25" true-dual (ie. NISMO) would be an ideal setup to retain most of the low-end/mid-range but also gains some ponies in the higher end.