For those looking to add spacers to their car... i've just put 15mm spacers all round on my F13 M6 comp and can report no adverse wheel wobbles or shaking... can't comment on the handling at the mo as I've not really pushed it hard yet, but it seems fine... I've not noticed the difference.
Pics below...
In the process...
The side that has NO SPACERS!
Side WITH SPACERS!
H