Yeah, I remember now. I know mine can run rough for a bit then smooth right out. Never timed it though. Just put motor mounts in and made a huge difference after the first start-up. They were shot after 50k miles. I will try to remember to put a timer on it next cold start.
You might try posting on the W212 section. There is a lot more traffic there and they deal with the M157 issues. Might be some insight to find there.
Good luck, I empathize with your predicament.

Reseated the injector and all seems good again.