Heh, actually one can jump from a designated speed to a little higher speed of burning but not at such high speeds. I happen to have a lot of CD-RWs operating at 4-10x but i burn all of them at 12x because Nero allows me to. I dunno why, might be my burning units (Teac CDW 540E, teac CDW 548E, Sont CRX210E1), but I feel fine with it. The speed increase is not so big and all, but it's shown as possible to raise speed a little.
I've heard of a guy able to burn 4-10 CD-RWs to up to 24x but he was using a hacked firmware, I guess made the unit unaware of CDRW's speed limit. i doubt alot of this story, but that's how it was told to me too.