If I understand correctly, this means that if someone puts a file above 5MB in their public directory and wants to use appcache, they will get this warning every time they run, with no way to suppress it. This is slightly problematic.

One thing would be to also pay attention to RoutePolicy, so people could explicitly opt some static files out of the cache, thus reducing the size.

Another thing that might be good would be to automatically exclude large files from the app cache. From your comment, it sounds like the failure mode is pretty bad if the cache gets huge.