You can buy any external SSD for bc/non enhanced games and because of the way the i/o works they are equally as fast as putting them in the internal.
Save the internal for Enhanced games that require it and use any old SSD for the bc games that don't
It amazes me how many ppl don't realize this works.
You can even 'store' SS|SX optimized games on external drives, and copy them over to internal when you need to play them.
I have a 1TB USB SSD attached to my SX. All the non optimized stuff resides there, including some enhanced games I know I Won't be playing any time soon but would rather not need to download the 50~60GB again. The copy/pasting doesn't take that long.