I almost pulled the trigger on this with my late iMac 2015 with 2TB Fusion. And I compliment you on a much easier solution to add a SSD with many less steps.
But my concern is I now have to manage 2 separate drives vs the current 1 drive. My 128GB blade gets 600+ Writes and 1600+ Reads, the new drive will be around 500GB/s R/W.
I did de-fuse my drives once when I first got it, but it was a bit of a pain using symlinks and I ended up fusing them back.
If I only had a 1TB fusion with the 24GB blade it would be an easy decision and I would not even use the blade.
What speeds are you seeing from your 840 Evo and how are you managing your file system with 2 drives?