An eight game playoff adds three more games to the season, which is a bunch. At most, I would think a four game playoff would be the max that could be managed, which is essentially would a plus-one following the BCS bowl games. If they went out to an eight team bracket, I would have the SEC demand (perhaps the Big 12 could as well) a bye because of the conference championship games. Then you could have six teams and the SEC would still add just one extra game.
All that being said, just leave it alone. It's rare to have this many undefeated teams. Heck, LSU won the NC with two losses...
RTR,
Tim