In order to exclude internal referrals you can add in the GA script some lines looking like:
I assumed that this list could just be added as a JS extension and the scope set as the Google Analytics tag.
However this does not seem to have much impact.
Is there a more efficient method via extensions?
Or should we modify the tag template and add the lines in there?
A standard JS extension won't work in this case because it runs before the tracker for Google Analytics is created. Since you'll need to do a template edit for that, I'd recommend contacting your account manager so we can help you set it up.