Throwing for 5,000+ yards is no easy task. He could do it, sure. But the odds are actually against him, not because Payton isn't here (I don't see that affecting Brees' play at all), but because it's only been done by a handful of players in the history of the league. Of course Brees IS the only player to have done it more than once.
I think the running game will have more of an effect this season. Grubbs is one of the BEST run blockers in the NFL.