You mean not confirmed for 10.13?Yes the Akitio Thunder 2 works with the RX 580 in 10.13.
Nvidia cards not yet confirmed to run natively.
However, up to macOS 10.12.x you can use Nvidia cards: https://egpu.io/setup-guide-external-graphics-card-mac/
You mean not confirmed for 10.13?
Will there be a throttling issue when using TB2?
So, I could buy a 8Gb 580 for around £310 plus a Akitio Thunder2 for £260, plug it into my late 2015 27” iMac and I’d be good to go?Yes, afaik so far only some RX 580 work out of the box in 10.13.
And no, the difference between Thunderbolt 2 and 3 is negligible: http://barefeats.com/bandwidth.html
So, I could buy a 8Gb 580 for around £310 plus a Akitio Thunder2 for £260, plug it into my late 2015 27” iMac and I’d be good to go?
Hmmm... maybe you're right!asfik this only works to drive an external display, not the iMacs retina screen