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Dell XPS 15 9550 Hard Drive Options?

I have a Dell XPS 15 9550 with the 32 GB M.2 PCIe for caching and a 1 TB HDD with my OS on it configuration and I'm looking to upgrade to faster SSD speeds. 

Am I better off just changing the HDD to a SSD or can I get even better speeds keeping the 1 TB HDD for storage and swapping the 32 GB M.2 PCIe (currently used for caching) to a bigger M.2 PCIe and putting my OS on that?
I was thinking of using a Samsung EVO 850 Pro if I swapped the 1TB HHD or a Samsung 950 pro if I swapped 32 GB M.2 PCIe.
I take it I would also need to alter the BIOS setting from RAID to ACHI when upgrading if I'm not mistaken. 
Any advice is welcome.
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