Midi is useless for WAR2, midi is the pure source of a sound or a total composition.
Midi is not even a sound, midi is only a kind of signals for a compositions, it's like the note of a composition.
It's usefull if i want to open an instrumental in share parts into a audio software for producer.
So i can have that instrumental in share parts like the drums, the piano, the violins etc, etc, it's mostly for dj or producer who want to make a mix of a sound or take independant parts of a composition.
I dont see why you want to use midi into war2 untill war2 have his own reader of midi and that sound to play the midi signals, and i really dont thing it's the case.
The only way you can save space is by finding a way to use mp3 with war2, if you cant the only second option is too compress your war2 combat into a zip file so people will save time on the DL.
PS: Also, quality is not a probleme, the quality of sound if pretty good even for now in 2015, dont forget blizzard use real orchestral and alot of high producing materials to do this game..
Your not gonna get better quality for the original sound of that game..
Trying to get better quality will mean to add plug in on your basic sound and will get bigger on the final save, and since we need to use wav, your wav gonna be the same or even higher.
The only thing you can do to give a new dimension to this game, its to create a new patch with new music, new special effect etc etc, it mean, do something totally new.