Having a hard time imagining the use case for this.
For consumer gear, almost any SSD sold today will be faster than someone would ever need. Just use that as a cache and save some money.
For pro/enthusiast gear, money would probably be better invested simply getting more RAM -- with 32GB, in many cases I have 20GB or more of that being used as a filesystem cache. Cache tends to very rapidly exhibit diminishing returns, to the point where I doubt I'd even notice an extra 32GB sandwiched between my RAM and SSD.
Maybe as a non-volatile cache for large bursts of writes?