drives I have internal are 4 15K SAS drives connected via MacRaid which with the new internal SSD connection will be faster.
Once you get past the limits of "internal" storage vs really fast external storage, you'll see that you really don't want a lot of internal storage because of the limits of power supply and heat reduction. With the massive external fast storage, you don't need to burden the internals of the workstation–which is a good thing!
I suppose this is a limitation of a MacPro. With PC's it's really not a problem to properly power and control 8 drives (32tb) in a well designed and cooled mid-size case, or up to 16-20 (64-80tb) in a full size case. I'm of the opinion that I'd much rather choose my own power supply (clean quality power) to supply my storage drives than let the OEM of a RAID/NAS chassis choose one for me. They tend to go as cheap as possible, where I tend to buy extra quality/power as a safety margin. By the same token I've seen precious few RAID/NAS cases that keep the drives as cool as a well designed case can. And then there's noise.
I have several NAS enclosures personally and I've built/supplied a grunch of then, and very few have equaled even an average PC case in power, cooling and noise. I have a new Synology 1813+ on pre-order I'm hoping breaks that mold. And I'm hoping that it's internal CPU/processing will relieve the main CPU of a bit.
With the more recent advent of RAID/NAS boxes for home and small business users we are tending to gravitate towards their use. With the traditional drives in a PC case most are used to we basically pay for drives, a bigger power supply, and a bigger case.. the last two if we don't already have them. With external RAID/NAS boxes there's more expense, and up until TB or a quality home hard wired network you're still hobbled by cheap power supplies, inferior cooling, more noise, and inferior transfer speeds. You can work past these things, but the costs add up rapidly.
Considering all that, there is no greater bargain or performance choice than an "in case" storage solution. More convenient in most cases too. Yet, NAS's and better home networks are giving us more choices.