Neil, i have the catalog with its previews, and the ACR cache for the images as shown in the develop module on a ssd, a samsung 830, and find it does improve performance.
today i would choose for a samsung 840 pro, most ssd have good read speed, only a few have good write speeds for large files. Samsung 840pro is one of them. ( samsung830 is no longer produced).
So my system config is: a 128gb ssd as systemdrive, holds nothing else; a 256gb ssd for my catalog and acr cache; my online image files are stored on 2 hdd's 2tb wd caviar black. When i import a shoot to develop it, i put these images on another 256gb ssd, and when done editing i move the folder with the images to the hdd.
I use mobo with z77 chipset and intel core i7 processor, with 16 gb memory. Little problems with speed in library module, only when having a lot of spothealing on portraits of adolescents fot instance.
N.B.: i have 1:1 previews, and when done editing recreate the 1:1 previews, does make a difference in library module speed.