Well, it's not the MAP sensor! After 2 trouble-free cold starts built up my confidence, today's cold start reverted to crap - and that was with the Firefly's MAP sensor in play.
So I immediately shut it down, fetched the Firefly's ignition coil and hooked
that up.
And it fired up perfectly! So I immediately shut it down and reinstalled the Metro's coil. And it fired up perfectly again! So much for A-B-A testing... I hate intermittent problems.
I'll leave the Firefly's coil in play. If it turns out not to be the solution, next up will be the distributor or just the pickup sensor inside.