Just for the record, after nearly a month of tests, last friday I decided I had enough and replaced the MR216i-p with an LSI 

prtdiag -v | grep -i lsi
4   in use    PCI Exp. Gen 3 x8 PCI-E Slot 4, Broadcom / LSI SAS3008 PCI-Express Fusion-MPT SAS-3 (mpt_sas)

The system has been working ok since then, with all the tests running, no lockups, no timeouts etc.

Sadly, the lmrc driver, while promising, is not ready yet.

Thanks to all who helped me and gave advice and to Hans and all those who wrote lmrc.

Regards.
Maurilio.