How do I recover my database after losing a tablespace with indexes?
How much improvement are you expecting from the SSD? It is surprising how little physical I/O actually happens on busy indexes and I don't think I've ever solved an actual performance problem with physical I/O devices.

I don't know enough about PostgresSQL to comment on buffering or recoverability and the like but I will say that choosing performance over recoverability is probably not a good practice. 

I never want to find out that I have found a previously unknown recovery bug when my data is sitting in a tape on a shelf. I learned the DBA craft in the days before RAID - every night was a sleepless night. Always, always, always use RAID's.  

If a user is complaining about performance, tell them you need a dozen SSD's to put in a 10 disk RAID5 array with hot failover. He'll leave with his head spinning but he might come back with a purchase order. 

Good luck.