I have a step-by-step guide on the procedure I followed. That is to break the fusion drive, repartition the drive and rebuilt the fusion. My performance is absolutely on par in the new setup. Bring your own time machine backup!
http://tmblr.co/ZeDCRyZsh-kC
Wow that's a pretty good guide, all done in the command line. They only delete half the logical volume which gives most people issues though... I will use that guide if i ever have to do it again but i will delete both parts of the logical volume. They also merge the final storage volume into the fusion drive, im not sure how the performance would be on that?