CD-RW's 10x can most times be burnt at 12x/16x if your writer supports this speed.
For 24x You will need a writer that supports 24x and some 24x CD-RW ultra speed rewritables.(16-24x)
Mitsubishi/CMC Magnetics/TDK do make them.
CMC Magnetics was the first one who made them according the standards.
Based on earlier rewritables I would recommend the Mitsubishi/Verbatim ones.
I allready have seen
Verbatim CDRW 16-24x but only 650 mb's ones. At my local stores for 3 euro's a piece (which is quite expensive since they sell Verbatim DVD+RW's for 4,5 euro's. But I don't have a DVD burner yet. Still waiting on what LG and Lite On will come up with. )
Online I have seen them at Opus/Fuerio website for arround 2 euro's.
So ther definetly out there. But I know there hard to find.