The 8 series is kind of one of those cars that everyone thinks is cool, but no one actually wants to own.. It makes for one heck of an autobahn cruiser, but honestly it's too heavy to be considered a sports car, too small to be considered anything useful in terms of hauling people, and too complicated and expensive for most people to want to even screw around with it.
After all, if you're gonna get an 8 series, why would you get an 840? Which means, you basically need to get the V12 ($$$$), and what the heck, you might as well try and find one in 6-speed. Those will be the ones to have, not many cars you can buy ANYWHERE for that kind of money that have a V12 and a 6MT, if that's what you're in to.
IMO, the 8 series will be one of those rare niche cars that will have a cult like following but generally disregarded by the public.
I don't think many of these super complicated BMW's will ever be highly collectible because most people wont be able to screw around with them. Combine that with the relatively low numbers of vehicles produced and low amount of aftermarket parts.
Think about the cars that are highly collectible now.... Most weren't particularly rare (although similar model variants and option packages are), and they weren't particularly technologically advanced for their time. I think much of that will continue on going forward.
1994 Infiniti Q45- First 5-speed swapped Q45.
2007 BMW M Coupe