How will the Radeon Pro 570 compare to the M395X?

[hotcheese]

In other words, what is the probable graphical performance of the 2017 low end 27" with a 570 compared to the top end 2015 27" iMac with a m395x?

 

[jerwin]

Look here:
http://creators.radeon.com/radeon-pro/

The m295x has 2048 stream processors (32 compute units) and is theoretically capable of 3.5 TFlops.
which would place it at the 570 level.

However:
1 Are you satisfied with the max from two years ago?
2 Will paying a premium for a better card now allow you do things that you wanted to do with a m295x , but couldn't?
3 Finally, is it more "cost effective" to use a external GPU?