LSI Have released a new firmware 23.18.0.0014 (MR 5.8 P1) Nov 26, 2013 and the release notes include the following fixes:-

SCGCQ00525647 (DFCT) - Cachecade 2.0 with MR 5.7 fw peformance is much lower than MR5.6
SCGCQ00552847 (DFCT) - Data Integrity issue found with CacheCade when CacheCade disassociation is initiated after running IO

I note they have also removed MR 5.6 & 5.7 from their download archive (the problem one and the preceding one). I resolved my problem by rolling back to MR 5.6 which seems to be stable with Cachecade write cache on.

Is anyone brave enough to try the new version and see if the problem is resolved in this? Not sure if I want to risk it on production servers after all the problems this caused the last time I upgraded.

Tim