how much work and effort should go into customizing a release for GitHub as compared to CPAN?

You're confusing deploying and releasing. You deploy changes (patches) to your module via GitHub, and you release versions (tarballs) to your users via CPAN.

Because GitHub is far more than just a git repository host, GitHub can technically be used to host releases as well, but noone does that —at least not exclusively— because cpan won't find them.

On the other hand, some people use GitHub as their bug tracker instead of rt.cpan.org. You can specify your preferred bug tracker in your release's (distribution's) META file.