A RAID 0 SSD will be nearly as fast and in my experience you are rarely running a RAM preview in AE and doing something else I/O heavy at the same time, so that saturation won’t happen in AE that often.
I have an OWC Accelsior here which I think is basically this.
I find that caching in the background is disk-bound with multi-layer, large-format comps. Playing back the composite preview is nowhere near as demanding as actually writing the individual caches for each layer-frame in the comp in the first place.