AS you know, the CFC just replaces the file by outputting exactly the same data as the file would. Without seeing HTTP traces, JavaScript error logs etc etc it's very hard to give an answer or see what is wrong. Please put your code online so I can take a look at it actually running.
Btw, my answer here http://forums.adobe.com/thread/1319717?tstart=0 refers to a script that uses Ajax already, and the data is JSON encoded too.