I drove an M2C a couple weeks ago, and here's my thoughts in comparison with my 2018 LCi...(both cars were US-spec). Mine is DCT, the M2C was a manual
- Sound: I prefer the sound of the N55. Deeper, and love the note at WOT. That said, the noise from the M2C was louder (stock for stock) and had a more enjoyable note than the M3/M4. Sound enjoyment is completely subjective and varies from person to person.
- Seats: The M2C seats are way better. Not enough of a reason for me to upgrade by itself. These are the seats it should have had from the outset.
- Power: Yup, there's more. It is noticeable, but so is the extra weight of the car. It still feels balanced and very manageable. At various loads it does pick up quicker while rolling, likely due to having quicker spool on two smaller turbos when compared to the twin-scroll single. It's nice to know the S55 can handle more power, but crank-hub mods and other upgrades are needed to make it reliable.
- Having the M1/M2 buttons is nice, but not a requirement. the OG is excellent right out of the box for static settings and only comfort, sport, sport+. The suspension is still non-adjustable.
- Track times: Worthless to 90% of us. We aren't professional race/test drivers, and most people will NEVER see that fast of a 'ring lap. It's nice to know that it's just as capable, and 9s is a lifetime in racing...but we don't live in that world. Most of us drive on real roads with speed limits, rules, and traffic. The occasional track day. This doesn't make/break a purchase for me. 8m on the ring is still very fast for a street car.
- 0-60 times: 0.1-0.2s faster. Great. 40 more hp and minimal gain here. Again, not enough reason to upgrade. In the real world with stop lights and speed limits...4.2s is more than good enough for me. The OG is is still faster 0-60 than just about any American muscle car short of a special-edition (i.e. ZL1, GT500, Hellcat, etc).
The new M2C is a great car, don't get me wrong. I just don't feel the need to upgrade. If I really want more power and better handling...a Dinan Big Turbo/tune levels that power, and the M-Performance coilovers are great. Adding all that still brings me in under the cost of the M2C.
I kind of compare it to upgrading your iPhone X to the XS...it's a "little bit" better. Is it worth dumping a ton of money just so you can have the latest and greatest? When the following year's model comes out...are you going to feel buyer's remorse again?
Edit: And to address a comment below, the longer you drive something, the more you acclimate to it. Differences become less noticeable and you adapt. I daily-drive my LCi, and the differences are immediately apparent. That being said, as you adapt over time, you learn where and how to exploit those differences.