You can change the maximum speed of your burner in the k3b settings under the devices section.
BUT k3b will actually have a look at what kind of media it is before burning and if it gets a valid response
will limit your speed. Also if the "driver" it uses (I use generic-mmc-raw) communicates with the drive and gets only
slower modes it will not burn at a higher speed, might be a limitation of that driver to communicate with the burner.
I found in general that CDs burnt at a lower speed are a lot more reliable on other drives especially if you use cheaper
media or want to store them over time. I prefer to burn at 12x, I found it's still reasonably fast and produces CDRs that
most CDROMs can read (especially linux bootable CDs can be fiddly with this).
^Might give you and idea what I'm talking about.