I'm looking for switching my imap server cert over to LE, too. Also using courier-imap under Gentoo.
Last year, when I created my imapd.pem certificate from a CAcert provided certificate, I followed the sequence:
cat myprivkey.pem myservercert.pem >imapd.pem
and following a Gentoo related How-To, I continued by appending a Deffie-Hellman value to the final certification file:
openssl gendh 1024 >>imapd.pem
Apparently, you don't do the gendh part.
Was I doing something something unnecessary that won't be necessary with the LE provided certificate, when following your approach?
What advantage (if any) would the DH part provide?