Optimization of HTTP traffic in WAAS is limited to TFO, DRE and LZ. It doesn't cache files in the way ACNS does. DRE will be the only form of "caching" of HTTP traffic in WAAS. For this to happen, the traffic will be required to pass through a pair of WAE's so that the auto-discovery process can complete.
If the traffic only passes through one WAE on the way out to the internet and not through the data center WAE, the WAE will place the connection in pass-through as an asymmetric client.
To get caching like ACNS, you can install ACNS 5.0.13 in a Virtual Blade and redirect the web-cache service to it.