I was able to book for 8 just 60 days before our mid-August trip - it's taken some perseverance for a couple of the ADRs, but all has worked.
However, we usually book at most one meal a day since we like to take mid-day breaks, and not be 'held prisoner' by our ADRs if the mood or circumstances change, and frankly it's too much food and too much money for 2 TS meals each day.
BOG pre-opening breakfast, BOG lunch, CP pre-opening breakfast, La Hacienda dinner in hopes of a view illuminations, early Raglan Road dinner during performance hours, Boma early dinner (that one was pretty much wide open).
Some hints - BOG lunch (and breakfast if they open it up for your dates in the coming months) if you can't get an 8, you can book in any combination of parties if the ADR times are very close together - it's seat yourself.
Sci Fi - the true cars only seat up to 6 - if you book a party of 8 - you will sit at 'cafeteria type' tables placed between a car front and back - you will actually not face the screen, but be sitting perpendicular to it. They may be able to break the party up and sit you in cars next to each other, but that place is usually fully booked out and they may not be able to meet the request. So, that ADR we just did 6 - 2 of us have just agreed not to go (and I think they were happy about it). If you book that as 2 parties of 4, you would be more likely assured of sitting in 'real' cars.
Beaches & Cream - no such thing as a table for 8 or anything over 4 adults (comfortably) - you will likely be separated anyway.
Maybe other people know of other places where the table configurations will effect the seating of a party of 8.