how on earth would first person be an issue for performance, you actually have less to render since half the player model is never visible.
First Person means things like building interiors, vehicle interiors, weapons/pickup items, NPCs and what have you need to be rendered at higher detail, and in some cases companies won't even have detailed these at all before First Person (vehicle interiors are usually very easy to cheat when the player can only see a part of the front).
Also, remember LoDs; when the camera is zoomed out in Third Person, more objects will use a lower LoD mesh. First Person usually requires more of the world to be at a higher LoD.
I'd also argue that First Person generally requires a much higher FoV than Third Person (that's a whole argument I'd rather avoid, though).
