I switched from an F10 550i to a 650i GC with comparable config and have 2cents worth of thoughts.
I am very pleasantly surprised regarding the handling of the GC. I test drove a 640i without AD/IAS thru winding road a month before taking delivery of mine (which has AD/IAS). The 640i GC felt a lot more agile and stable in winding back roads vs the F10. The GC also feels quite a bit smaller (it is physically larger) than the F10 in windy roads even without IAS. The steering feel is heavier in the GC. All-in-all, you feel that you are really driving a sport car in the 6GC vs a sedan-trying-to-be-sport F10.
6GC's suspension is noticeably firmer than the F10. I feel that the suspension is way better tuned in the 6GC. I have yet to experience any boat-like feeling in my 6GC. I had ACS springs and AD on my previous F10 550i which reduced most but cannot completely eliminate the boat ride feeling. And in the 6GC, I find myself using comfort+ a lot more when driving in the city and roads of poor surface, and just the normal (or rather called comfort now) mode for a bit more spirited driving. In the F10, I pretty much want sport mode suspension all the time......... All in all, 6GC suspension is what BMW should have done to the F10 suspension in the first place......
Very planted cornering as well.
650i vs 550i..... Well, the 650iGC not only gets the new N63tu engine which is 10% boost over the 550i's N63, the engine management is also tuned for a sporty response from factory. No comparison. 550i is a sedan. 650iGC is a sport car, eager to go when you step on it.
Here, I would rank 650iGC on par with the F10. However, the 650iGC does have a lot less space inside the car for passengers and storage vs the F10. Also, the seats in the 650iGC are not as comfortable as in the F10, even though I have comfort seats in both. The beat seats legrooms are fair int he 650iGC, but headroom is noticeable less. And the rear window is TINY even compared to my E93 M3 vert.....
And no auto-trunk close on the 650iGC is my major gripe with it.....
2011 E93 M3 Convertible AW/CR DCT/Brembo GT BBK/Michellin PSS/Morr MS10
2013 F06 650i GranCoupe Ruby Black/Dakota Cinnamon/IAS/DHP/Comfort Seat/Soft Close doors/Surround View/Full LED