The 1password keychain is of course encrypted[1] (I think[3] that dropbox syncing uses the older agile keychain format?). I personally sync my 1password keychain with icloud. Apple claims[2] that iCloud data is encrypted during transfer, as well as encrypted at rest. If I was more concerned about that aspect of it, I would probably just sync with wifi.
I do not use the browser plugin (personal preference).
[1]: https://support.1password.com/opvault-overview/