You are right, Channel0 is slots 1/3 and Channel1 is slots 2/4;
So with modded bios it's possible to change PL separately for Channel A & B .
It's better cause differents settings by defaut is frequent for this timing;
For example, if I run a hight FSB/DRAM ratio and lower frequency, I get PL 4 for channel A and PL 5 for channel B.