Is it possible to use a modified installer that hashes and then re-uses files that are not modified?
For example, if a Firefox update changes two files, firefox.exe and a .dll, why are all PortableApps users re-downloading the entire Firefox installer? you could just have the installer use an index of the updated installer files, CRC32 or some other hash to verify the already installed unchanged versions, then supply the updated files.
Also, if the altered portions of the updated file contents are very small, you could use something like ParPar to patch those, yielding even more bandwidth.