Raid0 for HDDs made good sense prior to SSD, as HDDs have plenty of space and contained OS, Programs and All files. Infact all my systemsfrom about 1999 on and prior to SSD were based on Raid0.
Raid0 improves sequencial performanc but does nothing for acess time and very little for 4 K random performance (Some improvement if short stroking is used). So while Raid0 did NOT do much for improving boot time and program load times, it did improve working with large file/data structures.
Along comes SSDs. Normally to small (costly) to load 1000s of the larg jpeg/bitmap photos and not going to stick many DVD (one .VOB = 1 gig), or Blueray that one file may be up to 40 gigs. SO the SSDs are normally used for OS + programs, and for Gamers some games. Here, it is the access time and random 4 K performance that is important - So not much to gain by using raid0. Exception would be if large game maps are loaded, But for this one exception not really worth it.
SSDs about 40+ times faster than HDDs. Typical HDDs access time is around 12.6 mSec and raptors probably around 4->9 mSec, SSDs are measured in tenths of a mSec. Random 4K performance for HDDs are terrible compared to an SSD.
Just as RPMs affect HDD performance, Size effects SSDs, That is a larger SSD is faster than it's smaller brother. This means that if you take two smaller SSDs (with their inceased Sequencial performance) will probably perform SLOWER in real life day-to-day usage than the same SSD but twice the size - Reason: The larger SSD will have higher throughput for the 4K ramdon read/writes and this is (again) what is important to a OS + Program drive.
Cashing a HDD with an SSD. Only makes sense if NOT using an SSD for OS + Programs. SRT was developed primarily for thoes that could NOT afford the larger SSD required for OS + Programs. In this case the "cache" primarily held the OS start up files + Most used Programs. Boot Time and program loads ALMOST equaled a SSD. For a storge drive it is very difficault to gauge benifit and would very widely between users.
My Idea for a great system is:
1) a SSD (128 Gig) of OS + Programs
2) a 2nd SSD (I use a 256 gig SSD for this) for A) My most often used data files (I think I can pick better than a software quess (Caching algorthium). B) game maps
3) HDD - What a Large storage vessal does best - Hold my files and used as a Backup for my 2 SSDs