Top 8 is perfect for 16 teams. AFL does it and it works well. I think the NRL should adopt a similar format to the AFL (1v4, 2v3, loser gets a second chance, 5v8, 6v7, losers eliminated).
Making it to a top 5 would essentially eliminate 3 sides who are equal on points with 5th, or are only 1 or 2 points behind, which with the way that Origin is scheduled, provides a disadvantage to those sides which are heavily represented. If you suffer a few injuries due to origin and lose a few, you can count your season over. 1v8 week one is ok, but I'd prefer the AFL finals format. Makes the games that much more interesting and provides build up, rather than 8th place having to face Mt Everest in week 1 of the finals.