Some are mutually exclusive. Unless you mix and match.
PCI-Express SSD blade for performance 1100MB/sec
PCIe SATA III for cheaper and less in performance. Even with two in RAID0 it isn't 1100-1500MB/sec of Express blades.
Slot 2 & 3 or 4 would allow for two cards or controllers (blade does not use a controller, just an adapter) / slots 3&4 share a bridge and bandwidth.
Put the audio libraries on one 'device' and go with whatever capacity you want from 250GB up to 2TB (2 x 1TB)
Put the system on the smaller but fast "blade"
And even 4TB hdd in the drive bays are no slouch.
Part of it is in proper configuration and making the most of separate bandwidth.
Blades can go from $275 for 256GB w/ adapter up to $700 for 1TB
Apricorn Velocity Duo ~$90?
Samsung 850 EVO or Pro SSD that can go on SATA II or III and $95 250GB units up to $390 for 1TB.
I like the XP941 256GB with adapter unit for the system $275, then use the Duo and 850 EVO 500GB ($189) and add a 2nd 500GB later if desired. Similar basically to the Apple/Samsung:
Samsung XP941 256GB PCIe 2.0 x4 M.2 SSD MZHPU256HCGL at Amazon
Apple Samsung 1TB PCIe SSD for MacBook Pro Retina iMac Mac Pro Late 2013 2014 | eBay
SATA Express meets the '09 MacPro - Bootable NGFF PCIE SSD - Page 26 - MacRumors Forums
Samsung XP941 M.2 (NGFF) PCIe SSD in a Mac Pro tower