Thanks rnd me
Upon testing, I have found that regular browser caching seems to be ok. It seems too many potential problems for me to use the manifest.
The main problem seems to be round trip time on mobile networks. A php that contains the word 'test' sometimes takes three seconds (I am in Aussie, the server is in the US). So I think I will look at CDN.
Thanks for your thorough explanation - it forced me to squeeze every optimization out of my code so that I could NOT use the manifest file.