[throwaway64]

1) yes, in fact more so, because they rely upon no fixed keys

2) there is zero reassurance, there is also zero reassurance gmail isnt sending all your mail to the NSA, etc. TOR helps you ensure you can keep you tormail and your clearnet identities as separate as possible, alternatively, run your own service.

[chimeracoder]

As stated in a comment above, using PGP/GPG could help. If those emails are encrypted, that adds an extra layer of security around the contents of those emails.

(Of course, that rests on the security of prime-number encryption, which may not be the best assumption when dealing with the NSA, but that's another discussion.)