Oh what a horrible task it would be and that is an understatement
converting all archives to 7z using a -r -t7z -mx9 -m0=LZMA:d80m:fb273:lc5:mc1000 would make files smaller.
I did a test run on like 500 MVP mods a close to 2 years ago (just RAR to 7z nothing else) and it would have saved an average of 58%
but it would take a solid week to do 3836 files (24/7)
but besides just re-compressing the files, there are many ways users could make mods smaller when they create them. For example DO NOT EAcompress files before archiving them up, do not include a whole BIG/IFF file when you can just include what was changed. You could use many different methods that would reduce mod sizes by a considerable amounts.
Lets take the last MVP stadium released, it is 32,173,662 to download now currently. It can be compressed down to 3,733,506 (by not ea compressing, only including what has changed, and using 7z). The file was reduced to 11.6% of its original size