bit depth is slowing you down? hmm. most video cards today are optimized for 32 bit color (4 bytes).

if you are trying to shave some cpu/gpu cycles... look into maxfpsbk and maxfps.

those 2 will yield you the most benefit

try maxfpsbk 15 or less and maxfps 30 or higher.
i normally recommend 15 and 40. if you still need to save more, tweak the maxfpsbk lower since you aren't looking at hit most of the time anyhow.