iMac + Fusion + Samsung 850 Evo - Fastest option?

[orangefridge]

Hi all,
I currently have a 27" Late 2015 iMac.
I have a 2tb Fusion Drive (installed) + Samsung 850 Evo 2tb (Laying around)

What would be the best configuration for speed in the imac?
1. Put Samsung SSD in and use as boot.
2. Make a Flash / SSD fusion if possible?
3. RAID the Flash / SSD together somehow?

Will the SATA connector speed limit the Samsung so much that it's not even worth putting inside?
I really don't want an external thunderbolt option as it seems expensive and I'm already shuffling things only having the x2 Thunderbolt ports.

Thanks,

Ryan