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If you are using a standard turbo, I would just go from that flex joint back.
2.25" is standard I believe, so 2.5" would be a good improvement. 3" would be a bit overkill unless you are hunting for big bhp with a bigger turbo
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from my limited diesel knowledge you want to get everything out of the turbo as quick as possible. So ideally youd have no exhaust but in reality the bigger the better. 2.5" is the right size to go for in reality.
What you want to go for is the decat, standard centre pipe and then remove the backbox and replace it with a straight pipe or a cut down centre pipe.
The manifold elbow part you posted a picture off doesnt need changing.
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All about 3" though I can't take credit for the 1 on the estate. It sounds epic
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Just a warning, if you go too big it can be counter-productive, quite aside from being pointless. Besides, the bigger you go the more expensive, heavier and more of a pita to fit it becomes. 2.5" has more than enough flow for stage 2.