The Instagram user "TheKlapper" gives a fun insight into some insane upgrades that the 65mm cameras went through in order to keep the production up to speed. One of the things that were developed just for the film was a 2000' magazine, which allowed them to shoot up to 17 minutes per roll!
As I mentioned earlier, the Sony F65, Sony F3, Canon C300 and Arri Alexa (in the earlier years where it delivered 2K footage) all do this :) Though the C300 can't deliver raw because of hardware restrictions in both outputs and on-board processing.
And I will correct myself by noting that you wouldn't downscale the raw footage before doing the VFX and compositing work anyway if you are delivering for 4K, but rather render the CG elements at that resolution and fit the 6k .r3d footage to a 4K project in-software. My bad!
It depends on how you downsample and export the 6K image to a 4K file. If you downsample using the Red codec you'll simply get a downscaled version of the footage, with the same compression. An uncompressed format is needed when you downsample, to gain the extra color data from the process.
The downside is that you'll end up with 4K footage that takes up a lot more disk space than the original 6K footage, which makes this process mostly viable only with heavy VFX work.
It's also useful to obtain what some might call a "true 4K image", since a 4K sensor does not always equal 4:4:4 color depth or a detail level representative of the resolution. Cameras like the Alexa, Sony F65 and F3, and Canon C300, all downsample from a higher resolution sensor in order to get the best raw footage at their delivered resolutions.
RED simply takes an alternative route by giving us access to the full resolution of the sensor, which we can downsample or, as you said, use for reframing and stabilizing purposes in post.