There should be a performance gain between the onboard solution and the use of a hardware RAID solution as you are going to have more dedicated resources for the RAID. You may find useful information about our RAID controllers here.
You might want to contact our pre-sales team to find out how much benefit you are going to get when using one controller over the other.
Would a controller like RS3DC080 pick up the existing RAID configuration from the C600 series onboard controller?
What are the benefits of choosing an Intel raid controller over a LSI one, when the hardware specs are the same?