I was afraid of a response like this, but makes sense.For hometheater the CCM7.5 will be a better speaker than the CCM682, however in situation with your 800 series speakers the CCM8.5D is a better match but a lot more money unfortunately. The CCM7.5 S2 would be a closer match than the CCm682 because you have the continuum driver. The CCm8.5D would be better because it has the Continum driver and diamond tweeter which the CCm7.5S2 does not have. The CCm682 has nothing similar to the 800 series speakers. If you read the Dolby White paper it clear states that in-ceiling Atmos speakers should be timber matched and full range speakers, it goes even further to say they should match your front speakers.
Oh after hearing my CWM8.3D in-walls i can completely advocate for the CI800 series line. They are truly amazing and match seamless with the D3 speakers....they are amazing! I wish i could do a complete CI800 surround and ceiling setup, however as with most things, some comprises had to be made with the theatre and the ceiling speakers were the ones.
I haven't really experienced much ATMOS/DTS-X content yet so not sure how much the ceiling channels will be utilized. I guess my question should be if the upgrade would be worth it with the amount of content and upcoming content out there. I think i will have to eventually upgrade though because the points you made about the similar materials between the CCM7.5S2 and 800/CI800 are enough for me to justify it.
Another question: Currently have SCMS as back surrounds, would you upgrade these to 805D3 before upgrading ceiling speakers from CCM682 to CCM7.5S2, or vice versa? Put another way; are the Height channels more important now than the back surrounds?