I made a reply earlier but it doesn't seem to have posted. I will repeat that thought:
I don't think you will ever get a better seal between the head/gaskets/manifold than you have now with new gaskets and proper torqueing. I would not loosen the manifold at this point. I don't think there is anything to gain if you have free movement of the downpipe.
While I have read that manifold bolts can be retorqued after several heat cycles that would be done one fastener at a time to prevent movement of the initial assembly position of the gaskets in relation to the other components.
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Any twenty minute job is just a broken bolt away from a three day ordeal
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