By 800MHz, do you mean DDR3-1600, or DDR3-800? Some motherboard BIOS' and such (CPUz does this too) report DDR3-1600 as 800MHz because the clock frequency of DDR memory (excluding GDDR5) is half of it's transfer rate, 1600MT/s, and memory manufacturers and such simply call it as 1600MHz to make it easier to understand.
If you mean lowering DDR3-1600 to DDR3-800 (actual frequency is now 400MHz and transfer rate is now 800MT/s), then don't do that. Like nna2 said, that would be detrimental to performance regardless of the timings because the bandwidth would then be insufficient.