I'm doing something similar, using Box.com for cloud storage. I didn't want to use Dropbox because it will sync my whole library on every computer that I have connected to it. I don't want my library on my work computer, for instance, so I use a mapped drive to the Box WebDAV url for my account. That makes Calibre move very slow, though. A compromise I started is to use a sync program to a local directory on the one or two computers I don't mind having my library on. I sync a local directory to my cloud library, work in Calibre locally for performance's sake, then sync again. The other good thing about that approach is that I control when the sync happens. With Dropbox automatically syncing it would be running in the background like crazy. That may not be a lot of load on the system, but it is necessary load.
If I need to, I can access the library directly on my tablet, for instance, through Box.com - so I don't need to depend on Calibre to move content to my reader. If I really want to, I can use Calibre directly against the mapped drive. That works very slow, and is not suitable for bulk edits, but small changes to individual titles works all right.