Even keeping the drive always 75% empty would be enough, but partitioning off is the easiest way to make sure it's never exceed 25-33% full (assuming the drive behaves like that in the first place).

To verify the drive uses the all of the drive as a cache, you can run full drive sequential write test (like the one in HD Tune Pro) and analyze the speed graph. If, say, a 480GB drive writes at full speed for the first 120GB, and then the write speed drops for the remaining 360GB, this means the drive is suitable for this kind of use.

I think controllers might've been doing some GC jobs to always keep some amount of cells ready for pSLC use, but it should be a few GBs at most and shouldn't affect the use case depicted here.

> Partitioning off a small section of the drive feels very 160 GB SCSI "Let's only use the outer sectors".

In that it was very reliable at accomplishing the goal?