No it is not good enough, because people install packages also just by wget'ing them and installing with dpkg -i.
MITM-attack for example on open WLAN-accesspoints is really easy. Also many kind of redirections and tricks can be made so although the user thinks he is getting something from some netsite with Firefox/Fennec/microB/wget/lynx, (s)he instead gets the file attacker has changed.

Also, if some package has been installed now from a repository, and it has checked with Release.gpg that it is in fact that very same package what repository maintainers have checked for.

If there is a compromise in the system and you would like to know what files have been changed, in RPM-based system you can just check authenticity and integrity of any installed package with "rpm -V", because there still is the package's GPG-signature available locally, and if you want to be real sure, you check the filesystem externally using 100% trusted non tampered tools.

With DEB-system, once some package is updated, it is much more difficult to check that files from the package you have installed are non-tampered. The Release.gpg file supports only the recent version of that package in the repository.

IN RPM-based system, lets assume you think wget has been tampered when your friend's friend had a 5 minute time root access when you were visiting toilet. Let's assume, in the repository wget has a new version already. But you want to know if wget was tampered on your local system.
You just run command: rpm -V wget
It will check wget-package against all the md5sums and against the GPG-signature which was embedded in the package when you installed it.