IMO 20 lootas are too much. 12 are just fine (4 hits with str7 is just fine to break a rhino or kill a dp)
biker boyz are not good. Too many points for not much to do. Nob Bikerz instead is a killing force.
If you get nobz on foot, put them in a trukk
Always take a painboy. Always 10 nobz, not less. Always different armory on them so that to alocate their wounds. Typical nob unit:
1 painboy waaagh! banner
3 nobz powerklaw. One gots ammo runt, the other boss pole
3 nobz big choppa. One gots ammo runt, the other boss pole
3 nobz choppa. One gots ammo runt, the other boss pole.
All nobz got 'eavy armour. All nobz got cyborg bodies.
If you want to make a nob biker unit, try this stats, and give them bikez.Remove 'eavy armour, as the bikez give them one. If you want to see their force get 2 units like this. Put some lootas with them (to open the rhinos/vindis e.t.c. and watch them win and not die at all. Your friends will hate you.)
For each nob unit, get a boss to make them troop. Boss is tunk hunter. He's also a killing machine (6x str10 attacks kills almost everything). Give him power klaw, eavy armour, cyborg body, squig. Always keep him in the unit.
You may think that those are too many points to spent in a unit. If you see this unit on stage, you will reconsider this. Always remember that as you got different kidz, they allocate wounds separately. (don't remove a kid for every 2 wounds. Just say "hmm, this one goes here, and the other one there).
Then you could get some shoota boyz. 30 boyz on foot are just 225poits, but every opponent will hunt them, instead of other units you got. You could always put them in a battle wagon. You may think that BS2 is not good, but think of the 60+(depends on the big shootas) shots you will fire. Something will die there mister. And then. 3 attacks in charge are not bad at all. You don't need the off-hand.
After all those, you could think of more special units. Everything in the ork codex is worth (well, almost everything) if you know how to play it.. This is why every good player is afraid of us, and this is why they call us broken-army. We just love to be broken.
(try a boomgun on a looted wagon. get a rhino you got, paint it red, and put a cannon on it. 43" range(with moving 7"), str8AP3. We just love it. Deff koptas outflanking with TL-rokkits. Skorcha buggies killing hordes for 45 points. e.t.c. Orkz are a treasure dude.. LoL)