I replaced the HDD with a Samsung 850 Pro (late 2015 iMac). Removing the "fusion drive" was not necessary -- if you install OS X from scratch, you can configure the two drives to work in parallel.
Confirming successful replacement of Core i5 with Core i7 6700K with the late 2015 model iMac. It is possible, but you need to be very careful when attaching the heatsink to the CPU -- it is possible to tighten the screws so tight that it damages the processor.
Once they have, you'll be able to view a graph of their reputation gained over time.
Here's a preview of what the graph will look like:
No reputation gained yet.