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The little I know about exhaust headers is that if they are not equal length then there's a chance one pulse is still creating pressure when another exhaust valve opens essencially creating more back pressure on the piston.
This is also a reason why it could be more efficient to turbo a 3 cylinder (or 3 cylinders per bank) rather than a 4 cylinder (or 4 cylinders per bank on a flat plane crank) because the pulses are more separate from each other and therefore do not affect each other. For the same reason cross plane cranks aren't the best idea when it comes to turbo charging.
Another thing is that different lengths could affect exhaust scavenging, although I'm not quite sure how that is affected with the addition of a turbo.
As far as a diesel goes, wraping the exhaust to insulate it is always a good idea. It keeps exhaust temps hot for the turbo and if you have any emissions equipment it helps keep that warm enough to do it's job too. With SCR it should help reduce the need to burn fuel in the exhaust as well.
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