I usualy burn 8x CD´s at 12 x, but they have to be a reliable brand (Usualy Mitsui!), and they won´t give any problem!
As for less reliable media, at 10x (Memorex, TDK,etc) they don´t give problems also!
But right now your question doesn´t have sense, because there´s already 16x CD´s on the market (TDK and Imation at least!)! If you buy them you´ll be able to burn at any speed!
My experience says you can burn CD´s at least 2x the top speed it is recomended without damaging the CD!
Better burn in Hell with some company than freeze in Heaven all alone