Hi Dave,
I use 3 SSD's (1 for OS - only 120GB) and 2x240GB for my more "intensive" sample libraries. I don't use symphonic cube, so I can't help there.
However, I did run some pretty intensive tests using LASS 1.5, EW, and VSE (all) with some very interesting results.
1./ Using Raid0 accross both drives yielded no performance benefit specifically when used for samples. My drives are SATA3 using the latest sandforce controller, and have a thru put (tested using blackmagic speed test) of > 500MB/s in and out. The drives are now back in my mac pro 3,1 (so running SATA2) and on seperate SATA busses.
2./ I was able to increase performance of my machine by seperating instruments accross the 2 drives. Not a huge amount, but it did improve loading times. A little trial and error, depending on what your template looks like.
3./ I didn't see ANY performance decrease when filling up the drives. This is for reading the drive only. writing slowed down as they neared full (which makes sense). I don't use OS trim support (I'm on 10.6.x) - but my drives are the OWC 6G drives, which have extra space allocated above the 240GB (16GB) in order to maintain performance over time. I now use both drives with around 220GB of samples on each one - which is better than I expected.
Not sure this answers your questions, but there might be some useful info in there somewhere.
B.