Sadly, you really can’t run RAID internally with the 2012 and newer iMac’s. The reason is they don’t have dual I/O ports which run at the same data rate which is a requirement for RAID to work well.
But you could use an external RAID drive subsystem via Thunderbolt2 or 3 (depending on the model)
The best option for I/O speed internally is going with the new iMac Pro there you have a dual channel all setup and is doing RAID 0.
Basically, you have the best iMac for internal RAID now! While it’s only SATA III (6.0 Gb/s) it still smokes! Save your pennies for the iMac Pro ;-}