A colleague found a lot of articles where people use a P212 controller successful with ESXi 4.x, so we used that in an HP ProLiant DL360 G7 and tried to use passthrough to one of the VM, which would be used as a storage node for some backup solution. This was all tested with the free hypervisor so far.
Configuration works without problems and the device can be handed over to the VM.
5.1 U1 -> PSOD on driver load for Windows 2008 R2 and Ubuntu 12.04 LTS, this version is plain unusable for passthrough.
5.1 -> Windows 2008 R2 first detects it and can even initialize it, then after another reboot it shows error 10 (failed to start device).
We even swapped PCI-E slots and all the other helpful articles out there, still the same behaviour.
Overall the 5.1 seems much more robust than the U1 version, no crash whatsoever.
Would another controller work?
Would VMWare support be able to fix it?
Is it worth to downgrade the servers to 4.x (could this be done via esxcli)?*
This is a chicken and egg problem 
Without providing a proper backup solution, these servers do not go into production and without being in production we will not get a licence to raise a ticket.
*not an option, because of the storage size and limits of vmfs3