UPDATE: problem solved (I think)

Codes pointed to lean mix errors- Have always suspected vacuum leak, and finally found it- pipe under supercharger had come loose from the clamp, but only in the bottom (where it was virtually impossible to see).

Re-fit everything with a new clamp, runs like a champ.

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'87 911 Carrera Coupe / '03 E53 4.6 Dinan