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Update: after an expensive but worth it service bill from 3R Racing/Performance, the source of the cold start trouble has been located. My car, a 1977 911S 2.7 sold in California, did not have a check valve at the fuel pump installed from the factory. For that reason, the long fuel line from the pump to the engine bay was down on pressure for every cold start.
Since a check valve was installed, the engine fires immediately upon cranking from cold. If you're having cold start problems, check to see whether your car has a check valve at the fuel pump at all!
Performance and driving characteristics have improved vastly since a huge number of vacuum leaks were rectified with new rubber. The car runs so well now: no idle hunting, no excessive rev hang, but no huge drop either, great warmup characteristics.
Notably, the car is much more eager off the line as I let out the clutch. Before, it wanted to die the moment I'd let out any clutch and I'd have to give it a lot of revs. I learned to drive stick on this car, and I thought I was going crazy because I would stall it all the time. Now, with no vacuum leaks and no stumble when I let the clutch out, it's as smooth as butter!
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