A scanner will tell you if the octane wag high enough. I would definitely run some Lucas injector cleaner and a couple tanks of premium. I never trusted mid grade in our n63 even though I noticed it's the only engine I've ever seen that has 89 vs. 91 as the minimum. You can also buy a couple bottles of octane boost to bump up the gas in the tank.

Injectors are very pricy on that engine you're lucky to have new enough engine they could replace one. On our 2012 it's an all or nothing.

I started a thread to determine if anybody got away with mid grade in the n63 and never found anybody that confirmed it was ok.

Rough cold start points to plugs/carbon. I'm surprised dealer didn't bring up walnut blasting.

__________________
2011 E70 • N55 (me)
2012 E70 • N63 (wife)