On Ubuntu and possibly Debian systems, you can install packages which contain images which are signed by Ubuntu's EFI key. For example,
linux-signed-image-generic-lts-trusty is one of these packages.
Say I have a fully-encrypted disk and an unencrypted boot partition, as is required. I use a PGP encrypted key file to decrypt my disk. What exactly does using these signed images get me in additional security? How trivial is it to generate a "signed" EFI kernel image which is compromised?