Addendum to discusion of the opening formula of Illuga saga
Gríðarfóstra, the use of the perfect tense isn't so rare in this
context after all. E.g.

Vísivaldr hefir konungr heitit (Vilmundar saga viðutan).
Darius hefir konungr heitit (Alexanders saga).
Artus hefir kóngur heitið (Nikulás saga leikara).
Vilhjálmur hefir kóngur heitið (Jarlmanns saga ok Hermanns).