Most home-quality NAS can't come close to saturating a 10Gbit link. First off, they come with 1Gbit connectivity, so unless you DIY it with another server acting as a NAS and source your own 10Gbit connections, it's not likely to be very performant. Even if you are a good DIY computer techie, you'll have to tune it and mess with it and be lucky to get a few hundred MB/s, much less than the 2500-3000 MB/s you get with the iMac Pro local Flash.
^^^ THIS!

[doublepost=1533748720][/doublepost]By the way, I run an iMac Pro with 1TB SSD. For data I use a 2TB Samsung T5. It's only 500-ish MB/s but that's sufficient for my data. I've been an Flash and SSD professional in my day job since 2009 so I've been all-flash for primary data for a while, with HDD as secondary (backup). I do have some exceptions, my past 2012 iMac was Fusion Drive, but I switched out to an external SSD for boot and primary data, Fusion for music/video type media, Synology NAS (HDD) for backup and basic file services. My Mac mini Servers are all converted to Fusion drive, with a large 512GB SSD portion and the stock 1TB HDD, so they work pretty darned well most of the time.
@AnonMac50 I would just expand with a Samsung T5 or maybe (at much greater cost) an external TB array and call it a day. Of course unless you are doing really active data manipulation on TBs of data constantly, then you are probably a pro and should upgrade to the better spec'd factory iMac Pro.