I have not heard 8k's ot 7k's in person but I do have a full LCR of 5k's, have a friend with a full LCR of 55's, and I have 6k's (see signature) in a setup. 6k's will change your life. There is a thing they do with sound that I haven't heard from any other speaker (again - I haven't heard 8k's) that not even other Meridians can do. As others have said - 6k's are the go-to out of your choices.
I will add that my 6k's benefited from toe-in and even tipping the heads down/forward (this is easy with the screw-feet thingies) made a huge difference in staging. There is a thread somewhere on here about that issue.

main room: 861v4, 2x6000L/R (24/96) 1x5500HC (24/96), 2x DSP33(Rears)
MS200/Roon, 598, Revox b710mkII tape deck, Revox b760 FM tuner, Revox b790 turntable
bedroom: 568.2mm, 3x5000(LCR) (24/96)