After having some performance problems during the migration of VMs from several FC datastores to FATA datastores and back, I did a performance analysis.
With the help of Duncan Epping from Yellow Bricks Blog and his Article about blocksize ( http://www.yellow-bricks.com/2011/02/18/blocksize-impact/ )
and some other users of the VMware Forums, I changed the blocksize on all effected datastores to match each other.
This led to an enormous performance boost during storage vmotion, as shown in the following table:
SVmotion of a 15GB VM on a HP EVA 8100 Array, containing 145x300GB FC disks in 1 diskgroup and 40x1TB FATA disks in a second diskgroup
As you can see, having the same blocksize on all datastores involved in a storage vmotion process greatly improves the performance.
Keep this in mind, when setting up new datastores, as even if you configure a large blocksize on a small datastore which could take a smaller one, this does not impact the performance at all.
The discussion about the blocksize and the performance was done here.
Thanks to Duncan Epping and ThompsG from the VMware forums for their fast and detailed help.