I'll preface this that I've never run a full-on Biker Mob.
In my experience Warbikes work best in small groups. Personally when I've used Bikers I've found they work great in groups of 6-8.
Interestingly I just threw together a list of Orks as follows:
Wazdakka
Warboss on Bike
3 Groups of 6 Warbikers w/ Nob, Big Choppa, Boss Pole
1 Group of 8 Warbikers w/Nob,
PK, Boss Pole
2 Groups of 7 Warbikers w/Nob,
PK, Boss Pole (this is where you'd put the Warboss and Wazzdakka.
That's a total of 42 Bikes for exactly 1500 points. Not bad. Although there's no Nob Bikers in there, and you probably want at least a small squad of them.
Amusingly while building this list I noticed that Wazdakka is exactly the same amount of points as a naked Riptide. For the sake of Mork and Gork the Orks seriously need a codex update.
EDIT - 42 bikes not 33