Baz,
Good to see another Queenslander on the forum. If you use a wheel spacer under 17mm, you will have to replace all of the studs with longer items to give the required number of threads to adequately retain the rim. Any spacer larger than 17mm will bolt to the hub and then the rim will bolt to the spacer. You need to be careful using smaller spacers as the orginal studs from the hub and the spacer bolts will protrude past the spacer. If the rim has a flush surface, you will not be able to use it with a spacer (you need recessed landings where the rim bolts up to the hub.
I find they improve cornering. Trade-off is unless you want to perform an on-vehicle wheel balance (and then never remove the rim), they impact on the vibration characteristics of the car.
Like all unsprung mass, the aim should be to reduce it as much as possible, but by doing this, we are increasing it, placing slightly higher loads on the drivetrain and supension (it could be argued that you will do this anyway with a different offset rim, as it will take metal to achieve the offset anyway).
I would suggest buying a TUV approved brand as this incidcates that they have been tested.
You will also have to be careful regarding the centre cap sizing, as when using the larger spacers, the hub-centric centre may foul the bore for balancing the wheel where the centre cap fits. They might also foul on the front wheel bearing carrier.
As above, the best option is wheels that do not require these. Next option is to use these.
I have run both 17mm and now 21mm spacers. With Dunlop Dizerra 285/35R18's the rears just touch the guards under compression (but running a fair amount of neg camber). The front tyres will just touch the inner guards on lock.
Wear on the tyres will also be an issue and you will have to play with camber and castor to ensure the full contact patch of the tyre is working.
