That is what the caching config in the HTTP binding is for. It polls the URL once every X seconds and caches the result. The Items poll the cached version every so often to get their value. So your service only gets hit once every so often, not once every so often times the number of Items.
In short, the HTTP binding already implements your memcache.
See Comprehensive Wunderground using HTTP Binding Example for a complex example of using the HTTP binding in this way.
You can call JSONPATH transform from Rules using the transform Action. But it is not needed in this case.
And I get all the way to the bottom and Vincent beat me to the answer. 