1. bigger isn't always better.
2. Similar to point number 3
3. 'upgrading' to stiff ass coil overs that may bottom out over bumps and cause the car to lose control...things like coil overs might be good on a nice smooth race track, but not so much so on an Australian road..\
4. No idea why they aren't legal, but I'm sure someone will have a sensible answer
5. Can't go from airbag to non airbag, but if its an older car without an airbag, the wheel has to be a certain diameter, and have a padded centre, then I'm pretty sure thats legal