Best location for Project files

You could put them anywhere but my SSD are 512MB and 256MB but my SATA RAID is 10 TB. I put all my projects on the RAID because it is huge compared to my SSD. That is why I made my RAID. I Have 5 Seagate Barracuda drives striped. If you only have two hard drives in a striped RAID 6K and 8K Pro Res, R3D, BRAW files might hit a bottle neck

Depending on the actual sustained read/write speeds of the various discs while Premiere is running ... any?

In your setup, I might tend to suggest looking at the RAID first, then the OS/programs disk, then the cache disk. Does it make any operational differences? If not, I might go with on the RAID.

mgrenadier has a great point about using the Desktop backed to a cloud account. Very safe that way of course.

Neil

This used to be a more important issues years ago when drives and computers were not as fast as they are today.

Someone here may have a better handle on the ins and outs of the issue but I'm not sure it really matters, but what is crucial is that your project file and your autosave folder do not live on the same physical drive...  

fwiw, I have a folder on my desktop that I store all my premiere projects in and I have dropbox setup to automatically mirror all these files on the cloud.  

This works for me because I'm often moving projects between different computers and this way I can always access the latest version of the project file.