I ran OS X on a 2600x under Linux + qemu and it actually compiles significantly faster than my loaded Mac Pro. I wish hackintoshing Ryzen wasn't such a hassle.
Again, all depends on what you're using it for. If your workflow isn't significantly impacted by TB2 bandwidth, go eGPU. Nvidia prices are nice right now. A 1080Ti + eGPU is half of what the d700s would cost.
Purchased my nMP with D700s and and 6 core a few months ago for a little over 2k from ipowerresale.com. I bought the lowest config possible CPU/Memory and upgraded both myself later. I also purchased their 3yr warranty for around $250. They have been great to work with and even allowed me to upgrade my internal ssd from 512gb to 1tb by just paying the upgrade price even though the transaction occurred several months ago - highly recommend. I have no affiliation other than being a customer.
In your case as others have suggested, I would sell your current system and purchase another with the config you are looking for. Also, if the rumors are to be believed you might wait a bit to see what the 2019 nmp looks like. At the very least it will have a further impact on current nMP prices.