if you are trying to improve your performance by dropping from 24 bit to 16 bit... i believe wow will put it back to 24 bit, if your display is in 24 bit color mode.

for performance... there are a couple of things to do.

1. on a single wow, drop your settings in-game and exit all the way out. this will force the settings to be saved and used by the other clients (drop shadows to almost 0. shadows are very processing intensive)

2. launch your wows using keyclone commands. set the maxfps and maxfpsbk (if you do not, EACH wow will try to get 999 fps). as a recommendation, set your maxfps to 42 or 50... and maxfpsbk to 5, 10, or 20. lower maxfpsbk == more cpu/gpu cycles saved.

3. use keyclone's maximizer. associate a region for each wow. **set the in-game resolution to 25% of the main area** by doing this, wow will render 25% of the pixels it is now, and then does a 200% blit, which is fast. the result is almost indistinguishable from the original

ie:
big area is 1000x500 and each other region is 250x125... right now your in-game resolution should be 1000x500.
my suggestion would be to reduce that to 500x250.

this, combined with maxfpsbk, will radically reduce your cpu/gpu requirements