Thanks! I am very fond of it already.
Thoughts vs C63:
1. The AMG is more aggressive, both in terms of engine note and driving feel. It is sharper to pick up, brake and turn in. But the GTS doesn't feel too soft, and sounds far nicer, to my ears. I now get what GT really means.
2. The ride in the AMG is much harder than the GT, even in sport mode. In non-sport, the GT feels like a limo in comparison. Even my wife noticed that (and, apparently, the GT seats are more comfortable for sleeping too...)
3. From inside the car, the AMG was somehow both louder and quieter. You don't get to enjoy the note as much (and the wind noise with the the windows open made it impossible to hear anything), but more tyre noise and general engine hum when cruising.
4. No question, the drop in torque and loss of one gear ratio is really noticeable in general use. I just need to get used to it and drive accordingly (if I'm trying to "make progress"). If I do that, the GT is as quick and the gearbox (yes, even the auto) is more engaging, but it requires more planning. The AMG allowed me to be really lazy and took off quickly in any gear, but that also made it less fun to drive in manual mode.
5. The interior is petty comparable in quality, but the GT feels more special (the stitching etc). There is actually more space in the back for the kids (6 and 4 year olds) too, and access is no problem.
6. GT infotainment is rubbish, but I knew that, and Android Auto on my phone, with a bluetooth usb/aux works perfectly, so I'm happy. The Bose system is better than the HK in the AMG. If I hadn't just blown my purchase budget, I'd be looking at one of the upgrade options that have come up on here....but that's for another year.
7. Touched on this above, but however good the AMG sounds from outside, the GT sounds SO much better from the driver's seat, and in my scientific test of driving through Petworth (high walled single lane town centre for those who don't know it) at the weekend it's louder too.
8. At the end of the day, the AMG is just a very nice C class. There's something undeniably cool about an estate car with a 6.2l engine but the GT is a supercar. For me the reasons for change were all about enjoying the looks, the sound, the rarity, and the whole ownership experience. Any performance car these days is quick, and if we're chasing 0-60 times, then we should be going electric.
@JonW Having had a 335i as well, the C63 was a much bigger step up from that than the statistics would suggest. You wouldn't have been disappointed, but the GTS is a very different car. I needed an estate with two babies, and it was a perfect car for the past 3.5 years, but I'm pleased with my decision to change (so far, pending the first big bill etc!)