I'm not entirely sure, but I think 32 bits are faster than 24 bits, because CPUs are 32 bits and since 24 bits is hell to read/calculate...

On 8bits I'm not sure, if your primary surface is 32/24/16 bits then I would have to say probably not, since well the 8bit bitmap colours have to be converted to the color settings of the surface/display, since otherwise it would look crappy, however if your primary surface is set to 8bits then I think it would be faster!