From what I know and searched using ramdisk for Sphinx/Manticore search won’t improve performance much because linux is efficient in caching disk and also SSDs are quite fast now.
However, when creating a large delta for ease of use, we noticed a 1 minute delta of 700 MBytes each minute would amount to nearly 1TB of writes per day.
So we decided to created a directory in /dev/shm and use it for only delta indexes while keeping main index on SSD.
We have no problems using this method and currently can recommend this method to decrease write level load on SSDs.
Hope this helps others.
If you have an opinion of this, feel free to give.