So, update on my shenanigans this weekend. I went to try an FF, and as I drive the dealers demonstrator rather than the one I was looking to buy, I suspect it was a lot better than the (much older) one I was looking at.
I was impressed, certainly enough to let my heart rule my head and pay a deposit on a car. However, after more consideration (a lot more than I normally give to such things) and getting back into my car, I've changed my mind

Whoops!
Firstly, I just cannot subscribe to the notion that the FF would make my car feel like a roller skate, or in any way inferior. It didn't, and if anything it highlighted just how good the AMG GT is, and how overpriced a new FF is!
Here's my thoughts on the car:
First thing's first, I was a little underwhelmed because the combination of a nervous salesman and traffic meant I only got to properly rev it twice in an hour's test drive. Also, I spent the hour trying to get used to the ergonomics....it's a challenge getting used to fixed paddles (when you're used to them moving) and no indicator stalks! I just couldn't really relax and get into it, but still, some underlying qualities still shone through.
The first thing that stood out was the sound. The lack of it

Outside the car it's a glorious wail. Very different to anything else I've driven, a typical Ferrari sound I guess. But inside, up to about 3k rpm it's fairly quiet - the car is VERY refined - but it does improve as revs rise. By the time it's shooting through 7-8k it's like an F1 car, but as I said, I only got to do this a couple of times.
Secondly, the ride quality. This was really unexpected but I thought it was one of the best riding cars I've ever experienced. Again, I can't see this as anything other than good, almost miraculous given how flat and sharp it drove. Very impressive, and I suspect due to the long wheelbase and what I imagine is fairly sophisticated suspension.
Steering. Simply AMAZING. Ferrari's seem to have a good rep for steering and I can see why. I really like my GT's very direct rack (although it's not to everyones taste) and the FF's was similarly direct but a touch heavier. The steering wheel is also fan-bloody-tastic....tiny little thing, nice and thick and with lots of ridges and bumps in the right places. It's an odd shape though.
Performance was hard to judge and despite being 150bhp up on the GT it really didn't feel like it in day to day driving. I guess because it only has about 20b/ft more torque and delivered much higher up so it needs working harder, which I didn't really get to do. On the couple of times I did use the full rev range I'd say it felt like it had very long legs (the car just keeps pulling) but faster than my GT or any other 5-600bhp car I've had? No.
The driving position is definitely different to what I'm used to though - the steering wheel is further away with my legs in a more bent position but this is mitigated by a steering wheel which comes out absolutely vertical so you get a consistent reach all round it. The seats are comfortable, but with way less side support than my GT....they'd be great on a long journey though.
Build quality?!?! Whilst the demo had NO rattles or unwanted noises and the material quality is phenomenal, it didn't feel any better than a top end German car and I'd say something like an S Coupe is way ahead. I got back in my car and it didn't feel in any way inferior.
Things I didn't like were fairly predictable - the sat nav is touch screen and utterly dire. Awful. And I'm sure it'll be worse on the 2012 example I'm looking at too. Then I'm back to the driving position - I'm only a size 8 but my foot kept hitting the underside of the dash (I later realised it was the position of the OBD port which is right in front of the brake pedal). It's also way off the mark compared to what I'm used to for technology.
So yes, it did have some fairly impressive qualities, despite the limitations of the test drive. As it should be for a £300k+ car (the spec on the one I was looking at was outrageous), but it definitely doesn't feel that kind of value if I was to judge it as 2-3x 'better' than other cars I've had. I guess I'm well into the territory of diminishing returns though, so whilst it was undeniably a special piece of kit with some aspects that are better than my GT (sound, ride quality and steering are all stand-out items), there are plenty of other areas where it's so far off the mark that I actually preferred it when I get back in my AMG.
Ultimately though, there were a couple of other nails in the coffin. Firstly there's the mileage I'll do - if I whack another 10k in the next 6-8 months then I'll have a 35k mile V12 Ferrari, which I suspect won't be the easiest thing to move on. Secondly, as I am merely a wage slave, an employee, and not the owner of a business, it's not a car one can use everyday so I'd end up getting annoyed at having to find way round NOT using it.
It's clearly a car one buys for a certain experience, for the brand and image. It's definitely not a car one buys because it is better than other much cheaper options. Dare I say it was a bit superficial
Maybe some will think that I just don't 'get it' (and maybe I don't) but quite frankly, after much consideration, I'll stick with my AMG thanks!