Hello. A common speed of  6GB/s will be negotiated as supported by the controller interface. I do see so that the PERC Seies-9 controllers support speed of 12GB/s.
Hi All,  Recently purchase some addtional drives for my R420 which has a PERC H710 installed with 6GB SaS drives.  the drives which i have been supplied are   HGSTUltrastar C10K1800 which are 12G SaS drives. i assume that these will function at the fastest level that controller can provide as the spec sheet from HGST just says 12G but always assumed SaS like SATA will  work with older controllers just at a lower speed. thanks 
Dell PERC H710 Mini Embedded (Poweredge R410)