I personally don't have problems with lag, but some of my friends, during rain will experience major FPS loss. I was thinking toggleable client-side rain, where mobs won't burn, ect but the player doesn't see the rain, making the(hopefully)best of both worlds.
Good suggestion, but A)It's not always up to date and B)Not everyone can install mods well(despite all the Youtube content)
optifine actually is up to date though - just download the 1.7.2 or 1.7.4 one ... if you use a normal launcher run the .jar by running a run.bat or similar ...or if you using magic launcher try putting it in mod folder, and then when you bring up your client, if optifine is enabled you should have an option to disable rain. alternatively I believe changing particle distance to minimal will make rain silent.
this is really not worth any time developing when there are solid client side mods already out there to do it.