Not a good idea. Someone I know implemented Cisco WAAS and is experiencing performance problems. Specifically the CPU on the WAE device is spiked to 100% utilized. They are attempting to per-position 80GB of data including large 1-2GB PSTs to the WAE (Wide Area Application Engine). In my opinion this is a recipe for disaster. First off Microsoft does not support PST access via network shares, and once you read why you will understand why caching PSTs is an even worse idea.
I ran my own little test by moving a PST to a share, starting wireshark (ethereal) and watching the activity. Accessing the PST over the network makes CIFS alone look very efficient.