Why do it at the block level (instead of tmpfs)? Or do you mean that you're doing actual real persistent disks that just have a lot of cache sitting in front of them?

The block level has two advantages: (1) you can accelerate access to everything on the whole disk (like even OS packages) and (2) everything appears as one device to the OS, meaning that build tools that want to do things like hardlink files in global caches still work without any issue.