If I keep my 2013 and go the external SSD route as the boot drive, are you talking about an enclosure that supports NVMe, but will attach to my iMac via TB1 or USB3.0?
But how would/could I run a TB3 drive?
I am currently booting from a TB3 drive on my iMac which only has TB1.
This can easily be done, but there are equipment and adapters needed.
The primary thing you need is the Apple Bidirectional TB3>TB2 adapter:
Effortlessly connect your Mac to a Thunderbolt 2 device with the Apple Thunderbolt 3 Adapter. Buy now at apple.com
www.apple.com
This is a pricey adapter ($50), but Apple is the only one that makes a bidirectional adapter. This enables you to use TB3 devices on your TB1 or TB2 Mac.
But, there is a catch, the adapter does not support power over the bus, so any TB3 devices you use much be powered from itself or a powered TB3 hub/dock.
If you get an NVMe TB3 enclosure that has its own power, than all you need is the Apple adapter. If you use something like I am using, the Samsung X5 NVMe TB3 external drive, than you will need something else to power it, such as a TB3 dock.
When it comes to the speeds, I see Write/Reads of 815/980 MBps with the original NVMe that was installed. I expected over 1000MBps read speeds, but I think the dock adds a little extra overhead which dropped my speeds.
If you can get an external TB3 drive that is self powered, then I suspect you would see even faster speeds.
And if I decide to open it up, I can opt to replace both the blade SSD *and* the HDD with, say, an OWC kit (although replacing the blade is a much more difficult operation).
Yes, you can replace both, or just one.
The 2013 has faster SSD blade speeds than my Late 2012 iMac. If you would replace the blade, you could see speeds over 700MBps.
Swapping the HDD with a SSD would see speeds about 500MBps.
In my Late 2012 iMac, the blade speeds are about the same as the speeds from swapping a SSD for the HDD. So there isn't too many reasons to replace the blade in the 2012, unless you want to put the two internal drives into a RAID0, which I might end up doing.
If I do that, I would expect speeds over 1000MBps.