You don't need the extra RAM kit, if you were to get 16GB, buy it as a 16GB kit, but you don't need it. It would help to know what you intend to do with the computer, i.e. if you were editing video across 6 monitors then you probably would need 16GB of RAM. But other than that it all looks good to me.
EDIT: You shouldn't have a problem booting off SSD in Raid 0 or keeping the smart cache there. Don't ask me how (I use one HDD QQ) but I remember reading from a reputable source about how to do it, so the info is out there.