There is a problem after the data.json was introduced. I've been using that excellent tool provided by 
beufreecasse but I had some fps issues because I could not base64 encode images at data.json.
Anyway, I'm working on a tool that replaces image data at data.json file. I'm encoding only the most critical images, because I can't find a solution for duplicate image data in data.json file.
Drop me a private message if you're interested of testing the tool
/timo