Of course, you're quite correct Graham about preload not affecting anything other than sag, but it's probably one of the biggest misconceptions in the motorcycling World. It astonishes me how many Dealers are ignorant too. I remember the look on a Dealers face when buying a new bike because it was advertised as having 'Fully adjustable suspension'. I asked for some discount, because the box of shock and fork springs in different Rates was missing. I wouldn't normally have set him up to look a twat, but he came out with the classic, 'you can stiffen 'er up a bit with these adjusters', when giving me a once over of the bike. Again, as you previously stated, different air gaps and different oils can make huge differences too.
One thing I would add, is once you find a source of suspension oil, stay with that Brand. Not all viscosities are comparable! If you do some Googling, there's a highly technical reason for this, which is way beyond my understanding. I called bullshit on this at first, thinking SAE standards meant the same - I was wrong